aura: Introduce ScopedWindowTargeter.

It is sometimes necessary to temporary replace the event-targeter for a window,
and restore the previous event-targeter after some time. To that end, this patch
introduces ScopedWindowTargeter.

BUG=318879, 338671
R=ben@chromium.org

Review URL: https://codereview.chromium.org/135543004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248101 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248101