Revert of Converts MessagePopupCollection into a long-lived object. (https://codereview.chromium.org/180513002/)

Reason for revert:
Possible cause of build breakage:

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/40180

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/40180/steps/message_center_unittests/logs/LeftPositioningWithLeftTaskbar

MessagePopupCollectionTest.LeftPositioningWithLeftTaskbar (run #1):
[ RUN      ] MessagePopupCollectionTest.LeftPositioningWithLeftTaskbar
Xlib:  extension "RANDR" missing on display ":9".
../../ui/message_center/views/message_popup_collection_unittest.cc:341: Failure
Expected: (r1.x()) \u003C (GetWorkArea().CenterPoint().x()), actual: 433 vs 400
[  FAILED  ] MessagePopupCollectionTest.LeftPositioningWithLeftTaskbar (430 ms)


Original issue's description:
> Converts MessagePopupCollection into a long-lived object.
> 
> This improves the behavior since there were several scenarios where
> observer methods would cause re-entrancy into the destructor of
> MessagePopupCollection. Since it observes MessageCenter, it knows when
> it should show popups and when it shouldn't.
> 
> This CL is based on dewittj's crrev.com/114553002 with a fix
> for some tests in linux-aura and win because of screen_aura.
> 
> BUG=327363
> R=dewittj@chromium.org, stevenjb@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=254101

TBR=dewittj@chromium.org,stevenjb@chromium.org,mukai@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=327363

Review URL: https://codereview.chromium.org/183883006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254145 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254145