Add default trybots for WebRTC try server.
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: 917f5d3ca488f358c4d40eaec14422cf392ccec9@5016 4adac7df-926f-26a2-2b94-8c16560cd09d

mwgoldsmith/libilbc,TimothyGu/libilbc,mwgoldsmith/ilbc,mwgoldsmith/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,mwgoldsmith/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,mwgoldsmith/libilbc,mwgoldsmith/ilbc,TimothyGu/libilbc,mwgoldsmith/ilbc,TimothyGu/libilbc