Add build link to rebaseline-o-matic CL descriptions.

third_party/WebKit/Tools/Scripts/webkitpy/common/net/buildbot.py

 # Copyright (c) 2009, Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 102 matching lines @@
 
     def _fetch_file(self, url_base, file_name):
         # It seems this can return None if the url redirects and then returns 404.
         # FIXME: This could use Web instead of using urllib2 directly.
         result = urllib2.urlopen("%s/%s" % (url_base, file_name))
         if not result:
             return None
         # urlopen returns a file-like object which sometimes works fine with str()
         # but sometimes is a addinfourl object.  In either case calling read() is correct.
         return result.read()
+
+
+def current_build_link(host):
+    """Returns a link to the current job if running on buildbot, or None."""
+    master_name = host.environ.get('BUILDBOT_MASTERNAME')
+    builder_name = host.environ.get('BUILDBOT_BUILDERNAME')
+    build_number = host.environ.get('BUILDBOT_BUILDNUMBER')
+    if not (master_name and builder_name and build_number):
+        return None
+    return 'https://build.chromium.org/p/%s/builders/%s/builds/%s' % (master_name, builder_name, build_number)