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

[chromium-wpt-export-bot]

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:

• Refine srcObject's MediaSourceHandle behavior w3c/media-source#306
• this relaunch also includes implementation of spec fix from
  Restrict handle getter visibility to dedicated worker 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 Add a reflection test for img.lowsrc. #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}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.