Add GetPreferredTrySlaves() to presubmit scripts.

BUG=23071
TEST=none
Review URL: http://codereview.chromium.org/251024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@27449 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index f7528e06..f42176d 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -143,3 +143,7 @@
   except IOError:
     pass
   return []
+
+
+def GetPreferredTrySlaves():
+  return ['win', 'linux', 'mac']