Revert of Fix for menus blocking user activity detection.

Revert of Fix for menus blocking user activity detection. (patchset #6 id:100001 of https://codereview.chromium.org/998603003/)

Reason for revert:
Linux ChromiumOS Ozone bot fails

Original issue's description:
> Fix for menus blocking user activity detection.
>
> Menus used to block detecting user activity. The fix works by making the
> UserActivityDetector a PlatformEventObserver rather than an
> EventHandler.
>
> BUG=462735
> TEST=ui_base_unittests --gtest_filter=UserActivityDetectorTest.*
>
> Committed: https://crrev.com/c655cc2a17c6c7fd113346bedc8433488c4b6dbe
> Cr-Commit-Position: refs/heads/master@{#320400}

TBR=sadrul@chromium.org,oshima@chromium.org,jamescook@chromium.org,afakhry@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=462735
Committed: https://crrev.com/4f634eb8e64c960b45fa30e7ea53da49951eec3b
Cr-Commit-Position: refs/heads/master@{#320424}

[Jun Mukai, 2015-03-13 01:48:17]

Created Revert of Fix for menus blocking user activity detection.

[commit-bot: I haz the power, 2015-03-13 01:48:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1006733002/1

[commit-bot: I haz the power, 2015-03-13 01:49:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-13 01:50:19]

Patchset 1 (id:??) landed as
https://crrev.com/4f634eb8e64c960b45fa30e7ea53da49951eec3b
Cr-Commit-Position: refs/heads/master@{#320424}