Change the app list folder management back button to a smaller size one(24x24).

BUG=318626
R=xiyuan@chromium.org
TBR=oshima

Review URL: https://codereview.chromium.org/68113010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234651 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234651