Add git cl format presubmit warning for extension and apps.

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
NOTRY=true

Review URL: https://codereview.chromium.org/153953011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249868 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249868