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This CL addresses an issue where KeyboardLock stops working (on
Windows) when the microphone permission is granted.  I tracked this
down to the RawInput key handler created by the UserInputMonitor
instance used in the audio pipeline (for keypress noise
cancellation).  After discussing this with the audio team last year,
we decided to pause the UserInputMonitor when keyboard lock is
engaged (and resumed when keyboard lock is disengaged).

In order for the UserInputMonitor to know when the keyboard hook has
been engaged, I've added a KeyboardHookMonitor class that the keyboard
hook impl can use to notify observers of keyboard hook status changes.

Change-Id: Ib8a51a0a49d33c6989c9325739d071c26860f98d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2615666Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843142}