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NativeEventObserver forwards will_run and did_run callbacks from the native
event processor to responsiveness::Watcher.

This CL also adds the macOS implementation. This requires modifying
BrowserCrApplication and ShellCrApplication to support a new protocol:
NativeEventProcessor, which allows observers to register for event will_run and
did_run callbacks.

Bug: 859155
Change-Id: Ie2db48efb4a93377ad54e91cbdb376d990b25f11
Reviewed-on: https://chromium-review.googlesource.com/1157235
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580345}