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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.

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

Change-Id: I11cee5d5279ad12524092518cadd05fff67bab72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519686
Commit-Queue: Shengsong Tan <sstan@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827122}