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When navigating, make the old RHWVMac dump its ui::Compositor in
TakeFallbackContentFrom, and have the new RWHVMac pick it up.

Ensure that the ui::AcceleratedWidgetMac preserve its contents through
this transition.

This is not enough to completely eliminate all flashes, because the
NSViews in play aren't being atomically swapped.

Bug: 829523
Change-Id: Ica6eab8dcea7a7987e8b7f83dbfd24acb2c6f7c4
Reviewed-on: https://chromium-review.googlesource.com/1023653
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552954}