Revert of Fix for menus blocking user activity detection (Retry 2).

Revert of Fix for menus blocking user activity detection (Retry 2). (patchset #1 id:1 of https://codereview.chromium.org/1024583003/)

Reason for revert:
Breaks multiple clicking on 2nd display

Original issue's description:
> Fix for menus blocking user activity detection (Retry 2).
>
> 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.*
>
> Committed: https://crrev.com/dbfa9102e7c6dfa13439ef1502d4baa9c3a31f31
> Cr-Commit-Position: refs/heads/master@{#321426}

TBR=jamescook@chromium.org,oshima@chromium.org,sadrul@chromium.org,afakhry@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=462735, 472222
Committed: https://crrev.com/a5784b85fe478bc4fd9ebc09514878154019fa79
Cr-Commit-Position: refs/heads/master@{#323394}

[spang, 2015-04-02 01:31:06]

Created Revert of Fix for menus blocking user activity detection (Retry 2).

[spang, 2015-04-02 01:31:21]

The CQ bit was checked by spang@chromium.org

[commit-bot: I haz the power, 2015-04-02 01:32:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1053773003/1

[commit-bot: I haz the power, 2015-04-02 01:34:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-03 20:21:05]

Patchset 1 (id:??) landed as
https://crrev.com/a5784b85fe478bc4fd9ebc09514878154019fa79
Cr-Commit-Position: refs/heads/master@{#323394}