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In order to ensure media files can enter fullscreen mode, manually
handle fullscreen states. Basically, a compositor sends configuration
events on each request for state changes from clients asynchronously,
which is unefficient for media files when they enter a fullscreen mode.

For example, when a youtube video file is being set to fullscreen, a
WaylandWindow is also set to a fullscreen mode, but the return value from
the WaylandWindow::GetPlatformWindowState still contains an old value until
the compositor calls with a configuration event. This short race is
enough for a media content to be placed in a forever "entering fullscreen"
state, which never results in a fullscreen state for media files.

Bug: 843018
TEST: XdgVersionV5Test/WaylandWindowTest.SetFullscreenAndRestore/0
XdgVersionV6Test/WaylandWindowTest.SetFullscreenAndRestore/0

Change-Id: Ib9bc8c0ce09412c36437d165efc6a0cad60c4d17
Reviewed-on: https://chromium-review.googlesource.com/1107636Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#569172}