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This change add a new event for desktop window focus state change.
With this change, ARC can have different behavior when Chrome window
or ChromeOS desktop get activated.

Fix the reverted change https://crrev.com/c/2519686

BUG=b:171450038
TEST=Active different window between Chrome/ARC and logcat events

Change-Id: Ied6245028578b89abb1ae376b7b49919c5cebee1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536578
Commit-Queue: Shengsong Tan <sstan@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827663}