add CompositorMsg::LoadStart, implement cef_load_handler::on_loading_state_change()

only adds the loading:true callback this time...

components/compositing/headless.rs

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