GitLab

As agreed on [1], media::mojom::MediaPlayer::SetMediaPlayerObserver()
is not the right method to expose in that interface since it should
be possible to register more than one observer for MediaPlayer related
events outside of the renderer process, so this CL migrates that old
method into a new AddMediaPlayerObserver() one.

[1] https://crrev.com/c/2526440/21#message-2189f6a0f6328b9cb01e5be9b4fae80ec7e1f080

Bug: 1039252
Change-Id: Ifb55f6c6eb896de02608df98654bc62ff7e7ee17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595049
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838690}