When underlyingSink.close() fulfills, assert state

Previously, WritableStreamDefaultControllerProcessClose checked the state was "closing" or "errored" when underlyingSink.close() fulfills. In fact it can't be anything else, so make it an assert instead.

index.bs

@@ -3405,7 +3405,7 @@ nothrow>WritableStreamDefaultControllerProcessClose ( <var>controller</var> )</h
   1. <a>Upon fulfillment</a> of _sinkClosePromise_,
     1. Assert: _controller_.[[inClose]] is *true*.
     1. Set  _controller_.[[inClose]] to *false*.
-    1. If _stream_.[[state]] is not `"closing"` or `"errored"`, return.
+    1. Assert: _stream_.[[state]] is `"closing"` or `"errored"`.
     1. Assert: _stream_.[[pendingCloseRequest]] is not *undefined*.
     1. <a>Resolve</a> _stream_.[[pendingCloseRequest]] with *undefined*.
     1. Set _stream_.[[pendingCloseRequest]] to *undefined*.

reference-implementation/lib/writable-stream.js

@@ -689,10 +689,7 @@ function WritableStreamDefaultControllerProcessClose(controller) {
     () => {
       assert(controller._inClose === true);
       controller._inClose = false;
-      if (stream._state !== 'closing' && stream._state !== 'errored') {
-        return;
-      }
-
+      assert(stream._state === 'closing' || stream._state === 'errored');
       assert(stream._pendingCloseRequest !== undefined);
       stream._pendingCloseRequest._resolve(undefined);
       stream._pendingCloseRequest = undefined;