WPT: Script child removal does not trigger execution

Given the old Chromium change that introduced this behavior:
 - https://source.chromium.org/chromium/chromium/src/+/604e798ec6ee30f44d57a5c4a44ce3dab3a871ed

... the old discussion in:
 - whatwg/dom#732 (review)
 - whatwg/html#4354 (comment)

... and the much newer discussion in:
 - whatwg/dom#1261
 - whatwg/html#10188

This CL upstreams an old test ensuring that removal of a child node
from a script node that has not "already started" [1], does not trigger
script execution. Only the *insertion* of child nodes (to a
non-already-started script node) should trigger that script's execution.

[1]: https://html.spec.whatwg.org/C#already-started

R=nrosenthal@chromium.org

Bug: 40150299
Change-Id: I750ccee0a2be834360c557042e975547c8ddd32c

dom/nodes/insertion-removing-steps/script-does-not-run-on-child-removal.window.js

@@ -0,0 +1,33 @@
+// See:
+//  - https://github.com/whatwg/dom/issues/808
+//  - https://github.com/whatwg/dom/pull/1261
+//  - https://github.com/whatwg/html/pull/10188
+//  - https://source.chromium.org/chromium/chromium/src/+/604e798ec6ee30f44d57a5c4a44ce3dab3a871ed
+//  - https://github.com/whatwg/dom/pull/732#pullrequestreview-328249015
+//  - https://github.com/whatwg/html/pull/4354#issuecomment-476038918
+test(() => {
+  window.script_did_run = false;
+
+  const script = document.createElement('script');
+  // This prevents execution on insertion.
+  script.type = '0';
+  script.textContent = `script_did_run = true;`;
+  document.body.append(script);
+  assert_false(script_did_run,
+      'Appending script with invalid type does not trigger execution');
+
+  const div = document.createElement('div');
+  script.append(div);
+  assert_false(script_did_run,
+      'Appending a child to an invalid-type script does not trigger execution');
+
+  // This enables, but does not trigger, execution.
+  script.type = '';
+  assert_false(script_did_run,
+      'Unsetting script type does not trigger execution');
+
+  div.remove();
+  assert_false(script_did_run,
+      'Removing child from valid script that has not already run, does not ' +
+      'trigger execution');
+}, "Script execution is never triggered on child removals");