Added a WebRTC test which will run for 15 seconds and measure key metrics.

The purpose of the test is to detect fluctuations in bandwidth numbers
and detect unexpected changes.

BUG=301581
R=holmer@chromium.org, xians@chromium.org

Review URL: https://codereview.chromium.org/25025003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227902 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227902