Add a gesture handler which toggles overview when swiping from the top with three fingers.

BUG=263528
TEST=Swipe down from the top of the screen with three fingers to toggle overview.

Review URL: https://codereview.chromium.org/26664008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228186 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228186