Move WindowPositioner to ash/wm
 WindowPositioner doesn't depend on chrome at all.
 window_positioner_unittest remains c/b/ui/ash as it depends on browser. I'll add separate tests in ash for this.

BUG=272460
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/25852004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226941 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226941