Added gsutil to depot tools

third_party/gsutil/oauth2_plugin/oauth2_plugin.py

+from boto.auth_handler import AuthHandler
+from boto.auth_handler import NotReadyToAuthenticate 
+import oauth2_client
+import oauth2_helper
+
+class OAuth2Auth(AuthHandler):
+
+  capability = ['google-oauth2', 's3']
+
+  def __init__(self, path, config, provider):
+    if (provider.name == 'google'
+        and config.has_option('Credentials', 'gs_oauth2_refresh_token')):
+
+      self.oauth2_client = oauth2_helper.OAuth2ClientFromBotoConfig(config)
+
+      self.refresh_token = oauth2_client.RefreshToken(
+          self.oauth2_client,
+          config.get('Credentials', 'gs_oauth2_refresh_token'))
+    else:
+      raise NotReadyToAuthenticate()
+
+  def add_auth(self, http_request):
+    http_request.headers['Authorization'] = \
+        self.refresh_token.GetAuthorizationHeader()