ui/app_list: Add git cl format presubmit warning.

Upon patch upload 'git cl format' is run on the diff and outputs a
warning if the diff has style violations, saving manual review
time for coding style.

BUG=None
TEST=None
R=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/169003002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251724 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251724