Gesture recognizer must be a singleton accross all aura::RootWindows

BUG=279317

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=229753

R=sadrul@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/25350006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229796 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229796