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When exiting picture-in-picture view, the associated wl_surface is not
destroyed, but attaches a nil wl_buffer. When restarting
picture-in-picture mode, skia_output_device_buffer_queue does not know
it has to reschedule the background image, resulting in
picture-in-picture being unresponsive.

Save the background buffer and re-attach it in wayland_window s.t. the
buffer queue does not need to schedule/damage background every frame.

Change-Id: I882e73cecd083d56e360ba61662db9eae566d199
Bug: 1142825
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519607
Commit-Queue: Kramer Ge <fangzhoug@chromium.org>
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Maksim Sisov (GMT+2) <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#825052}