Add enough Webdriver support to enable Get().

This is incomplete in several ways:

* It assumes that there's only one constellation (i.e. top level browsing context), ever.
* The session support is very basic indeed (no capabilities)
* Passing channels over channels may not sit well with IPC
* The error handling is mostly missing

components/msg/constellation_msg.rs

@@ -218,6 +218,8 @@ pub enum Msg {
     ChangeRunningAnimationsState(PipelineId, bool),
     /// Requests that the constellation instruct layout to begin a new tick of the animation.
     TickAnimation(PipelineId),
+    // Request that the constellation send the current root pipeline id over a provided channel
+    GetRootPipeline(Sender<Option<PipelineId>>)
 }
 
 // https://developer.mozilla.org/en-US/docs/Web/API/Using_the_Browser_API#Events