Update git_cl to look at right Tree Status URL for Chromium OS.

Improves output of git cl tree to use Chromium OS status, not Chromium Status.

BUG=chromium-os:11449
TEST=Output of 'git cl tree' for Chromium OS includes output from Chromium OS instead of from Chromium.

Review URL: http://codereview.chromium.org/6312047

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@73292 0039d316-1c4b-4281-b951-d872f2087c98

git_cl/git_cl.py

@@ -12,6 +12,7 @@ import sys
 import tempfile
 import textwrap
 import upload
+import urlparse
 import urllib2
 
 try:
@@ -1177,7 +1178,8 @@ def GetTreeStatusReason():
       print >> sys.stderr, 'Please install simplejson'
       sys.exit(1)
 
-  json_url = 'http://chromium-status.appspot.com/current?format=json'
+  url = settings.GetTreeStatusUrl()
+  json_url = urlparse.urljoin(url, '/current?format=json')
   connection = urllib2.urlopen(json_url)
   status = json.loads(connection.read())
   connection.close()