GitLab

This is a retry CL for https://codereview.chromium.org/1024583003/ with
a slight change.

UserActivityDetector used to be an EventHandler triggered when the pre-
target event handlers are triggered. When a menu is present, the
MenuEventDispatcher doesn't trigger pre-target handlers and hence user
activity is not detected.
With this change the UserActivityDetector is made a PlatformEventObserver
to make sure it's always triggered.

R=sadrul@chromium.org, jamescook@chromium.org
BUG=462735
TEST=ui_base_unittests --gtest_filter=UserActivityDetectorTest.*

Review URL: https://codereview.chromium.org/1074543002

Cr-Commit-Position: refs/heads/master@{#324366}