Revert of Fix for menus blocking user activity detection (Retry). (patchset #1 id...

Revert of Fix for menus blocking user activity detection (Retry). (patchset #1 id:1 of https://codereview.chromium.org/1013003002/)

Reason for revert:
Issue 468442 has resulted from this CL. A fix at a higher level must be done before this CL can be relanded.

Original issue's description:
> Fix for menus blocking user activity detection (Retry).
>
> Original CL reverted here: https://codereview.chromium.org/998603003/
>
> 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=oshima@chromium.org,jamescook@chromium.org
> R=sadrul@chromium.org,alexst@chromium.org
> BUG=462735
> TEST=ui_base_unittests --gtest_filter=UserActivityDetectorTest.*
>
> Committed: https://crrev.com/00e8cbeb43eb3cc99ffed3bc29a8a539dfece0a6
> Cr-Commit-Position: refs/heads/master@{#321066}

TBR=sadrul@chromium.org,spang@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=462735

Review URL: https://codereview.chromium.org/1016173002

Cr-Commit-Position: refs/heads/master@{#321199}