Add Skip Ad button to Picture-in-Picture window.

This CL adds 'skipad' to the Media Session actions available to web
developers. It allows them to use it to show/hide a Skip Ad button in
the Picture-in-Picture window. When users clicks it, the action handler
is called.

Screenshots: https://imgur.com/a/fxBNTKf

Change-Id: I017421e2efe9a8b31fd577647c1e2b5f73cb23f0
Bug: 910436
Reviewed-on: https://chromium-review.googlesource.com/c/1393331
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#622306}
31 files changed