Provide webrender_api::RenderApiSender to ScriptThread and DOM Window

This will allow the HTMLMediaElement later to get a handle to the
RenderApi for rendering video frames.

At a later time, all media handling should be moved to its own
thread/process that is communicated with via IPC. At that point this
can be removed again.

Original-patch-by: Sebastian Dröge <sebastian@centricular.com>

components/constellation/pipeline.rs

@@ -517,6 +517,7 @@ impl UnprivilegedPipelineContent {
                 webgl_chan: self.webgl_chan,
                 webvr_chan: self.webvr_chan,
                 webrender_document: self.webrender_document,
+                webrender_api_sender: self.webrender_api_sender.clone(),
             },
             self.load_data.clone(),
         );