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This CL adds a blur view to the HostView when the app becomes inactive
(transitioning to background, etc.) and removes it when the app becomes
active again.

Screenshot: https://drive.google.com/open?id=1UrWMUc-j0rRS-DQJ5zLLZ4_dTGgLohVr

Bug: 791112
Change-Id: Ic66c57c939bb607ad7b740cb8112bd449cd20d45
Reviewed-on: https://chromium-review.googlesource.com/807402
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521567}