Force break git cache locks when CHROME_HEADLESS is set.

BUG=406669
R=dnj@chromium.org,agable@chromium.org

Review URL: https://codereview.chromium.org/500903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291590 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -1221,6 +1221,9 @@ want to set 'managed': False in .gclient.
     if cache_dir:
       cache_dir = os.path.join(self.root_dir, cache_dir)
       cache_dir = os.path.abspath(cache_dir)
+      # If running on a bot, force break any stale git cache locks.
+      if os.environ.get('CHROME_HEADLESS'):
+        subprocess2.check_call(['git', 'cache', 'unlock', '--force', '--all'])
     gclient_scm.GitWrapper.cache_dir = cache_dir
     git_cache.Mirror.SetCachePath(cache_dir)