Enables stacked tabs for aura-metro

Ifdefs weren't quite right.

BUG=315195
TEST=run chrome (aura) in metro mode, touch the tabstrip and move tabs
     around, make sure tabs enter stacked mode.

R=ben@chromium.org

Review URL: https://codereview.chromium.org/60353003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233425 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233425