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We add extensions to WaylandDisplayObserver called
WaylandDisplayObserverExtensions that allow display information to be
sent through other protocols and be followed by a wl_output "done"
event.

WaylandDisplayObserver only reported scale changes to its observers.
Since remote_output needs to know about work area changes as well as
scale changes, we modify WaylandDisplayObserver to report all metrics
changes to its extensions, which can decide if it needs to send any
updates.

BUG=b:165094142
TEST=exo_unittests
TEST=Verify insets are updated in ARC++.

Change-Id: I6577fe15eb5b09527008cf69a26b5eec7a7f1374
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363481Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Prabir Pradhan <prabirmsp@chromium.org>
Auto-Submit: Prabir Pradhan <prabirmsp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800229}