Add virtual webcam check step for WebRTC factory.
Add virtual webcam check step for WebRTC factory.

The factory property was added for these builders in
https://codereview.chromium.org/14631018/
but since they use a different constructor than ChromiumFactory,
the step was never added to the builders in the client.webrtc
waterfall.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23222003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@217983 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build