Fire page visibility events in Picture-in-Picture

This CL makes sure page visibility events are fired even if a video is
playing in Picture-in-Picture. This doesn't affect "disable rendering"
optimizations as those are turned off  already if media player is in
Picture-in-Picture.

https://github.com/WICG/picture-in-picture/issues/110

Bug: 917303
Change-Id: If3788ce89f55e22af59bc69a6a52cd2a3309c518
Reviewed-on: https://chromium-review.googlesource.com/c/1400803
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622002}
3 files changed