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This cl adds ui::UserActivityObserver and
viz::mojom::VideoDetectorObserver to listen to relevant events when
generating idle events.

See https://docs.google.com/document/d/123eFd0rUTu0140V6hJKkkWmpqibdIfM-zTtUR5a2qaQ/edit#
for more detail.

Bug: 784232
Change-Id: I64bae608ea7500406197c3219f26599962256adc
Reviewed-on: https://chromium-review.googlesource.com/780480Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Renjie Liu <renjieliu@chromium.org>
Commit-Queue: Jia Meng <jiameng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519864}