No more headless compositor. Just the normal one.

This changes headless operation to strictly be a runtime option, rather
than a compile-time one. Note that the old headless version still relied
on a display server to support WebGL, while it now requires one all the
time.

Fixes #8573

components/script_traits/lib.rs

@@ -365,6 +365,8 @@ pub enum ScriptToCompositorMsg {
     TouchEventProcessed(EventResult),
     /// Requests that the compositor shut down.
     Exit,
+    /// Allow the compositor to free script-specific resources.
+    Exited,
 }
 
 /// Whether a DOM event was prevented by web content