Avoids centering docked windows, Unsnaps when centering.

BUG=322196

Review URL: https://codereview.chromium.org/79023009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237143 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237143