Add default trybots for WebRTC try server.

Today, our tryjobs default to run on all trybots since
we don't have any default list configured in PRESUBMIT.py.
Because of this, the --testfilter argument doesn't work
unless you also specify --bot when sending the tryjob.

With this CL, it is possible to use --testfilter without
--bot.
It also gets the benefit of excluding unnecessary bots
when doing platform-specific changes.

Most of the code is copied from Chromium's src/PRESUBMIT.py:
https://code.google.com/p/chromium/codesearch#chromium/src/PRESUBMIT.py&l=1030

TEST=tested submitting a tryjob with git try -t compile.
BUG=none
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2619004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5016 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed