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

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}

[afakhry, 2015-03-19 18:25:18]

(Retry) The exact same original CL after Oshima prevented marking events created
from the same native event as "repeat".

[afakhry, 2015-03-19 18:58:15]

The CQ bit was checked by afakhry@chromium.org

[commit-bot: I haz the power, 2015-03-19 18:58:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1024583003/1

[afakhry, 2015-03-19 20:04:41]

The CQ bit was unchecked by afakhry@chromium.org

[afakhry, 2015-03-19 20:10:10]

The CQ bit was checked by afakhry@chromium.org

[commit-bot: I haz the power, 2015-03-19 20:10:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1024583003/1

[commit-bot: I haz the power, 2015-03-19 20:40:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-19 20:41:24]

Patchset 1 (id:??) landed as
https://crrev.com/dbfa9102e7c6dfa13439ef1502d4baa9c3a31f31
Cr-Commit-Position: refs/heads/master@{#321426}

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

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1053773003/ by spang@chromium.org.

The reason for reverting is: Breaks multiple clicking on 2nd display.