Fix gcl try after PRESUBMIT files have been modified for multiple tryserver support

Note that other waterfalls are not guaranteed to support triggering via HTTP
or SVN. That's why this hardcodes tryserver.chromium.

BUG=334892

Review URL: https://codereview.chromium.org/197553009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@256947 0039d316-1c4b-4281-b951-d872f2087c98

trychange.py

@@ -376,14 +376,14 @@ def _GenTSBotSpec(checkouts, change, changed_files, options):
                                         options.issue,
                                         options.patchset,
                                         options.email)
-    trybots = presubmit_support.DoGetTrySlaves(
+    trybots = presubmit_support.DoGetTryMasters(
         change,
         checkouts[0].GetFileNames(),
         checkouts[0].checkout_root,
         root_presubmit,
         options.project,
         options.verbose,
-        sys.stdout)
+        sys.stdout).get('tryserver.chromium', [])
     if trybots:
       old_style = filter(lambda x: isinstance(x, basestring), trybots)
       new_style = filter(lambda x: isinstance(x, tuple), trybots)