Refactored how WebRTC content browser tests talk to their javascript.

We will now use the domAutomationController rather than looking at the
page title. This will confer the following advantages:

- Tests will fail right away when the js detects an error, rather than
  timing out.
- We will get a lot better stack traces when errors do occur.
- The communication path to the tests becomes more flexible with
  possibility for custom return values (for instance if we need to
  verify something in the C++ code or have a multi-stage test).

BUG=None
NOTRY=True

(doing notry since CQ gets stuck on telemetry test on mac_rel)

Review URL: https://codereview.chromium.org/190563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255795 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255795
Add multiprocess test helper functions.

Don't force people who want to write multiprocess tests to derive their
test fixture from MultiProcessTest.

TBR=phajdan.jr@chromium.org

Review URL: https://codereview.chromium.org/189373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255796 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255796