setMediaKeys(): move state changes and promise resolution to queued t…

[xhwang-chromium]

Fixes #585.

The setMediaKeys() algorithm in Section 7.2 of the EME specification performed several state mutations while running "in parallel", which violates the standard web platform concurrency model.

This change moves observable state changes, internal flag updates, and promise resolution inside a queued task to ensure they are executed on the main thread.

---

Preview | Diff

[xhwang-chromium]

@marcoscaceres Could you please take a look to see whether makes sense? I mechanically did this change (with the help of AI) to make sure matches the existing spec. Could you please take a look to see if it makes sense?

Here's a more readable summary for easier discussion:

   5. Run the following steps in parallel:
       1. If (CDM already in use or UA unable to use it):
           1. Queue a task to:
               1. Set attaching media keys to false.
               2. Reject promise with QuotaExceededError.
           2. Abort these steps.

       2. If (mediaKeys attribute is not null): [This is the existing association]
           1. If (Removal not supported):
               1. Queue a task to:
                   1. Set attaching media keys to false.
                   2. Reject promise with NotSupportedError.
               2. Abort these steps.
           2. If (Cannot be removed currently):
               1. Queue a task to:
                   1. Set attaching media keys to false.
                   2. Reject promise with InvalidStateError.
               2. Abort these steps.
           3. Stop using the CDM and remove the association.
           4. If (Preceding step failed):
               1. Queue a task to:
                   1. Set attaching media keys to false.
                   2. Reject promise with the appropriate error.
               2. Abort these steps.

       3. If (mediaKeys is not null): [This is the new association]
           1. Associate the CDM with the media element.
           2. If (Preceding step failed):
               1. Queue a task to:
                   1. Set mediaKeys attribute to null.
                   2. Set attaching media keys to false.
                   3. Reject promise with the appropriate error.
               2. Abort these steps.

       4. Queue a task to run the following steps: [Final Success Path]
           1. Set the mediaKeys attribute to mediaKeys.
           2. Set attaching media keys to false.
           3. Resolve promise with undefined.
           4. If mediaKeys is not null, run the Attempt to Resume Playback If Necessary algorithm.