Fix root layer naming in the compositor

The term "root layer" is used in the compositor to refer to both the
pipeline root layer and the page background layer. This can be quite
confusing. Instead, call the page background layer the "base layer,"
which is always the first child of the pipeline root layer.

components/compositing/headless.rs

@@ -98,7 +98,7 @@ impl CompositorEventListener for NullCompositor {
             // we'll notice and think about whether it needs a response, like
             // SetFrameTree.
 
-            Msg::CreateOrUpdateRootLayer(..) |
+            Msg::CreateOrUpdateBaseLayer(..) |
             Msg::CreateOrUpdateDescendantLayer(..) |
             Msg::SetLayerOrigin(..) |
             Msg::AssignPaintedBuffers(..) |