[Interop 2026] Advance MediaStream currentTime during playback (#62270)

Track elapsed playback time for MediaStream-backed media elements instead
of deriving currentTime from the latest received media frame. This keeps
currentTime advancing while the element plays even when a remote sender is
inactive, and preserves the accumulated time across pauses.

Correct the WPT helper so its audio cases create an audio track, and add
unit coverage for play, pause, and resume behavior.

Bug: None
Change-Id: I6e718c08395aaab82586b5a4225c0f029d8b8e84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8309752
Reviewed-by: Johannes Kron <kron@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1687921}

Co-authored-by: Michael Reeves <michael.reeves077@gmail.com>