Adding default trybot names to PRESUBMIT.py.

With this change, only the listed trybots will be the ones that a tryjob
is sent to. This works because there's a check in depot_tools/presubmit_support.py that looks for a function defined with the name 'GetPreferredTrySlaves'.

This makes it possible for us to add new trybots that will not receive jobs by default (only when the --bot flag is specified).

This CL is needed before we add the following try bots:
linux_memcheck
linux_tsan
linux_asan

BUG=none
TEST=submitting jobs to a local try server, with and without the --bot
flag.

Review URL: https://webrtc-codereview.appspot.com/939008

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3031 4adac7df-926f-26a2-2b94-8c16560cd09d

PRESUBMIT.py

@@ -159,3 +159,11 @@ def CheckChangeOnCommit(input_api, output_api):
   results.extend(input_api.canned_checks.CheckChangeHasTestField(
       input_api, output_api))
   return results
+
+def GetPreferredTrySlaves(project, change):
+  files = change.LocalPaths()
+  if not files:
+    return []
+  # Default trybots to run jobs at (if --bot is not specified).
+  return ['win', 'win_rel', 'mac', 'mac_rel', 'linux', 'linux_rel',
+          'android', 'android_ndk']