Fix leak in BackButton
 It wasn't setting the border just created.

BUG=155363
TBR=msw@chromium.org

Review URL: https://codereview.chromium.org/136143005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245046 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245046