chromite_wrapper: Support git submodule checkouts.

BUG=None
TEST=Ran locally

Review URL: https://chromiumcodereview.appspot.com/14585006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@198184 0039d316-1c4b-4281-b951-d872f2087c98

support/chromite_wrapper

@@ -37,6 +37,7 @@ def _FindChromite(path):
   roots = (
     ('.repo', 'chromite/.git'),
     ('.gclient', 'src/third_party/chromite/.git'),
+    ('src/.gitmodules', 'src/third_party/chromite/.git'),
   )
 
   while path != '/':