Add check to verify tree is open to PRESUBMIT.py.

This will disallow commits when our tree is closed.

BUG=chromium:342743
TEST=ran git cl presubmit with an open tree (no error). Then I closed the tree at http://webrtc-status.appspot.com and ran it again, got this message:
Tree state is: closed

***************
Tree is temporarily closed (testing presubmit hook real quick)
http://webrtc-status.appspot.com/current?format=json
***************

R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/8609004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5542 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index c7f3cc5..b029780 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -152,6 +152,9 @@
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasTestField(
       input_api, output_api))
+  results.extend(input_api.canned_checks.CheckTreeIsOpen(
+      input_api, output_api,
+      json_url='http://webrtc-status.appspot.com/current?format=json'))
   return results
 
 # pylint: disable=W0613