Fix SelectedNetworkController default metamask networkClientId (#1757)

## Explanation

Currently, the SelectedNetworkController does not set a value for the
default `metamask` domain on initial `NetworkController:stateChange`
event because it does not count `selectedNetworkId` as actually changed
in the patch yet. This manifests itself in extension with the
SelectedNetworkMiddleware setting undefined for the
`req.networkClientId` attribute. ~~This PR adds one more check to the
event listener to ensure that the first `NetworkController:stateChange`
event is used to populate the networkClientId for the default `metamask`
domain.~~ See resolution in comments

## References

* Fixes MetaMask/MetaMask-planning#1370

## Changelog

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### `@metamask/selected-network-controller`

- **FIXED**: `setNetworkClientIdForDomain()` will now ignore the passed
in domain value and set the networkClientId for the `metamask` domain
instead when the `state.perDomainNetwork` flag is false (default)

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate

---------

Co-authored-by: Alex Donesky <adonesky@gmail.com>

packages/selected-network-controller/src/SelectedNetworkController.ts

@@ -148,7 +148,11 @@ export class SelectedNetworkController extends BaseControllerV2<
     networkClientId: NetworkClientId,
   ) {
     this.update((state) => {
-      state.domains[domain] = networkClientId;
+      if (state.perDomainNetwork) {
+        state.domains[domain] = networkClientId;
+        return;
+      }
+      state.domains[METAMASK_DOMAIN] = networkClientId;
     });
   }
 

packages/selected-network-controller/tests/SelectedNetworkController.test.ts

@@ -21,30 +21,31 @@ describe('SelectedNetworkController', () => {
   });
 
   describe('setNetworkClientIdForDomain', () => {
-    it('can set the networkClientId for a domain', () => {
+    it('sets the networkClientId for the metamask domain, when the perDomainNetwork option is false (default)', () => {
       const options: SelectedNetworkControllerOptions = {
         messenger: buildSelectedNetworkControllerMessenger(), // Mock the messenger
       };
       const controller = new SelectedNetworkController(options);
-      const domain = 'example.com';
-      const networkClientId = 'network1';
-      controller.setNetworkClientIdForDomain(domain, networkClientId);
-      expect(controller.state.domains[domain]).toBe(networkClientId);
+      const networkClientId = 'network2';
+      controller.setNetworkClientIdForDomain('not-metamask', networkClientId);
+      expect(controller.state.domains.metamask).toBe(networkClientId);
     });
 
-    it('can set the networkClientId for the metamask domain specifically', () => {
+    it('sets the networkClientId for the passed in domain, when the perDomainNetwork option is true ,', () => {
       const options: SelectedNetworkControllerOptions = {
         messenger: buildSelectedNetworkControllerMessenger(), // Mock the messenger
       };
       const controller = new SelectedNetworkController(options);
-      const networkClientId = 'network2';
-      controller.setNetworkClientIdForMetamask(networkClientId);
-      expect(controller.state.domains.metamask).toBe(networkClientId);
+      controller.state.perDomainNetwork = true;
+      const domain = 'example.com';
+      const networkClientId = 'network1';
+      controller.setNetworkClientIdForDomain(domain, networkClientId);
+      expect(controller.state.domains[domain]).toBe(networkClientId);
     });
   });
 
   describe('getNetworkClientIdForDomain', () => {
-    it('gives the metamask domain when the perDomainNetwork option is false (default)', () => {
+    it('returns the networkClientId for the metamask domain, when the perDomainNetwork option is false (default)', () => {
       const options: SelectedNetworkControllerOptions = {
         messenger: buildSelectedNetworkControllerMessenger(), // Mock the messenger
       };
@@ -55,7 +56,7 @@ describe('SelectedNetworkController', () => {
       expect(result).toBe(networkClientId);
     });
 
-    it('when the perDomainNetwork feature flag is on, it returns items other than the metamask domain', () => {
+    it('returns the networkClientId for the passed in domain, when the perDomainNetwork option is true', () => {
       const options: SelectedNetworkControllerOptions = {
         messenger: buildSelectedNetworkControllerMessenger(), // Mock the messenger
       };