Accept subject template for mailer.

mailer.py

Index: mailer.py
diff --git a/mailer.py b/mailer.py
index 3fc72b13e532b97d18d76be1fa61fe24170a600a..978d22ff26095451931970df30ce7d0ef8ebbd79 100644
--- a/mailer.py
+++ b/mailer.py
@@ -167,12 +167,15 @@ class Email(BaseHandler):
     from_addr = from_addr_prefix + '@%s.appspotmail.com' % (
         app_identity.get_application_id())
 
+    subject_template = build_data.get('subject_template')
+
     recipients = ', '.join(build_data['recipients'])
 
     template = gatekeeper_mailer.MailTemplate(build_data['waterfall_url'],
                                               build_data['build_url'],
                                               build_data['project_name'],
-                                              from_addr)
+                                              from_addr,
+                                              subject=subject_template)
 
 
     text_content, html_content, subject = template.genMessageContent(build_data)