Mac dock App List anchoring code uses AppListPositioner.

The main logic for doing the anchoring calculations is now shared with
the Windows code.

BUG=317944

Review URL: https://codereview.chromium.org/69983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237547 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237547