Create CachingRietveld to automatically cache results for presubmit checks.

Multiple presubmit checks may call the same function multiple times, so it's
worth caching the results to speed up the presubmit check run.

Convert presubmit_support, git-cl and gcl to use it.

R=dpranke@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11280143

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@169726 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/git_cl.py b/git_cl.py
index 2a57a8a..6031862 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -742,8 +742,8 @@
     """Returns an upload.RpcServer() to access this review's rietveld instance.
     """
     if not self._rpc_server:
-      self._rpc_server = rietveld.Rietveld(self.GetRietveldServer(),
-                                           None, None)
+      self._rpc_server = rietveld.CachingRietveld(
+          self.GetRietveldServer(), None, None)
     return self._rpc_server
 
   def _IssueSetting(self):