GitLab

Original CL reverted here: https://codereview.chromium.org/1013003002/

Nothing has changed from that CL above. We are relanding after Oshima
prevented marking events created from the same native event as "repeat"
in this CL https://codereview.chromium.org/1017323002/

Menus used to block detecting user activity. The fix works by making the
UserActivityDetector a PlatformEventObserver rather than an
EventHandler.
Also made sure that PlatformEventSource has an instance in
ozone_platform_test.cc

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

Review URL: https://codereview.chromium.org/1024583003

Cr-Commit-Position: refs/heads/master@{#321426}