Implement Msg::Status with serialized url upon mouseover

Credits for Mike Blumenkrantz (@zmike), I just rebased against trunk and
fixed the url serialization.

Fixes #6178.

components/compositing/headless.rs

@@ -97,6 +97,7 @@ impl CompositorEventListener for NullCompositor {
             Msg::AssignPaintedBuffers(..) |
             Msg::ChangeRunningAnimationsState(..) |
             Msg::ScrollFragmentPoint(..) |
+            Msg::Status(..) |
             Msg::LoadStart(..) |
             Msg::LoadComplete(..) |
             Msg::ScrollTimeout(..) |