MSE-in-Workers: Relaunch with flag guards and handle visibility restr…

…icted

This undoes the previous launch revert commit 887a8c4e3d5203b0528e7c911b24be3aa67d2b4e, and includes multiple fixes.

Switches the two RuntimeEnabled features that enable MSE-in-Workers
from experimental to stable status:
  MediaSourceInWorkers: Basic support, still using legacy object
    URLs for attachment
  MediaSourceInWorkersUsingHandle: Upgraded to match updated MSE spec, final PR linked below.

Updates the stable webexposed expectations for both the Main/Window
context and the DedicatedWorker context.

MSE spec PRs with handle usage refinements:
  * w3c/media-source#306
  * this relaunch also includes implementation of spec fix from
    w3c/media-source#317

Intent-to-ship=https://groups.google.com/a/chromium.org/g/blink-dev/c/FRY3F1v6Two

Versus the original launch, this relaunch:
* adds base::Feature (aka flag-guards) for each of the two
  RuntimeEnabledFeatures, above (with same feature name strings).
  This is to comply with mandatory flag-guarding PSA process update,
  and is meant to mitigate possible binary respins in the event this
  feature pair yet again needs to be disabled.
* updates the MediaSource.handle attribute getter to fix the
  regression responsible for the previous revert, complying with
  MSE spec that was fixed to prevent that regression (see PR #317
  linked above):
  * Removes NotSupportedError exception throwing logic from the
    handle attribute getter.
  * Restricts visibility of the handle attribute to only dedicated
    worker contexts (removes visibility of it from the main/Window
    context versus previous launch attempt), along with corresponding
    webexposed stable web_tests expectations matching this change.
  * Updates mediasource-worker-handle.html web_test to no longer
    expect NotSupportedError exception, nor even ability to access the
    handle attribute of a MediaSource object on the main/window
    context.

BUG=878133

Change-Id: Id34a07254b9b98e79c495429f8ed79555b0c4580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3910706
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Mike Taylor <miketaylr@chromium.org>
Reviewed-by: Will Cassella <cassew@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1053854}

media-source/dedicated-worker/mediasource-worker-handle.html

@@ -34,15 +34,17 @@
 }, "Test main context receipt of postMessage'd MediaSourceHandle from DedicatedWorker MediaSource");
 
 test(t => {
-  assert_true(window.hasOwnProperty("MediaSourceHandle"), "window must have MediaSourceHandle visibility");
+  assert_true(window.hasOwnProperty("MediaSourceHandle"), "window must have MediaSourceHandle");
+}, "Test main-thread has MediaSourceHandle defined");
 
+test(t => {
   // Note, MSE spec may eventually describe how a main-thread MediaSource can
   // attach to an HTMLMediaElement using a MediaSourceHandle. For now, we
-  // ensure that the implementation of this is not available.
-  assert_throws_dom('NotSupportedError', function() {
-    let h = new MediaSource().handle;
-  }, 'main thread MediaSource instance cannot (yet) create a usable MediaSourceHandle');
-}, "Test main-thread-owned MediaSource instance cannot create a MediaSourceHandle");
+  // ensure that the implementation of this is not available per current spec.
+  assert_false(
+      "handle" in MediaSource.prototype,
+      "window MediaSource must not have handle in prototype");
+}, "Test main-thread MediaSource does not have handle getter");
 
 if (MediaSource.hasOwnProperty("canConstructInDedicatedWorker") && MediaSource.canConstructInDedicatedWorker === true) {
   // If implementation claims support for MSE-in-Workers, then fetch and run