Add WebRTC support

Reuse the MediaCaptureDevicesDispatcher from the chrome layer,
pretty much as is, and wire it in with WebContentsDelegateQt and
WebContentsAdapter/WebContentsAdapterClient for API delegation.

We also need to ensure that our user agent string mentions Chrome and
the Chrome version we're based on, in order to please websites that detect
feature support that way.

Change-Id: I0ddf8cd34e4add96bc36f59adfe8e0384e728d93
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>