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Whenever the ui::Layer for BridgedNativeWidgetMac is hidden, also lock
the ui::Compositor ("lock" in this sense means "disable drawing new
frames").

This ensure that we won't draw any blank frames (which is what would
result in a blank frame).

Bug: 712244
Change-Id: Ia8c17742e6a48190b9a3aead6443102a585d1f7d
Reviewed-on: https://chromium-review.googlesource.com/1129320
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574187}