Add more tests to Chromium WebRTC testers.

Add more tests to Chromium WebRTC testers.

These are tests that normally don't execute in the main waterfall
(except content_unittest).
The PyAuto tests are in the progress of being converted to browser_tests
and Telemetry tests, but we'd like to get them up here now so we can
retire our own machines running these tests.

Overview of the new tests added:
* PyAuto AppRTC: tests that the WebRTC reference application is
  working.
* PyAuto quality: performs video and audio quality analysis to catch
  regressions.
* content_unittests: subset of the main waterfall execution of this test.
  Used to catch regressions with recent WebRTC revisions in the
  chromium.webrtc.fyi waterfall.
* manual browser_tests and content_browsertests: Tests that normally
  don't run in the main waterfall, as they require audio/video devices.

Prerequisites:
The quality tests needs ffmpeg 0.11.1 and zxing
(https://code.google.com/p/zxing/) executables to be installed in the
PATH of the buildbot tester.
This will be done by:
* Win: Copying ffmpeg and zxing into build_internal\tools
* Mac: Installing ffmpeg into /usr/local/bin and copying zxing into
  ~/slavebin
* Linux: Installing ffmpeg into /usr/bin and copying zxing into
  ~/slavebin

TEST=local execution on Linux, Windows and Mac.
BUG=244550
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=208672

[kjellander_chromium, 2013-05-28 19:53:06]

Add more tests to get a complete coverage similar to the one we have today in
our internal WebRTC waterfall.

We're currently working on converting the PyAuto tests, but we'd like to get
these tests up running here now, so we can retire our old machines.

[Mike Stip (use stip instead), 2013-05-28 20:14:31]

lgtm

[commit-bot: I haz the power, 2013-06-26 08:35:44]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/kjellander@chromium.org/16145002/4002

[commit-bot: I haz the power, 2013-06-26 08:36:39]

Change committed as 208672