Sometimes options won't have a 'shallow' field. Don't freak out.

BUG=406535
R=stip@chromium.org,vadimsh@chromium.org

Review URL: https://codereview.chromium.org/504473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291502 0039d316-1c4b-4281-b951-d872f2087c98

gclient_scm.py

@@ -796,7 +796,7 @@ class GitWrapper(SCMWrapper):
   @staticmethod
   def _UpdateMirror(mirror, options):
     """Update a git mirror by fetching the latest commits from the remote."""
-    if options.shallow:
+    if getattr(options, 'shallow', False):
       # HACK(hinoka): These repositories should be super shallow.
       if 'flash' in mirror.url:
         depth = 10