Picture-in-Picture: resolve exitPictureInPicture() promise when closing window.

Previously, the promise was resolved before hitting the IPC to the
browser process. This adds an ACK IPC that notifies that the operation
is done.

Technically, this isn't checking that the window is really closed but
instead the ACK is sent after `Close()` is called.

Bug: 806249
Change-Id: Ibcae8789275f743a827340d0f22485484cbac5b1
Reviewed-on: https://chromium-review.googlesource.com/1042572
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: François Beaufort <beaufort.francois@gmail.com>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559247}
22 files changed