Auto merge of #24408 - servo:jdm-patch-31, r=Manishearth

Synchronize WebGL layer creation with underlying GL APIs

I believe the underlying cause of #24373 is that the XR callbacks can end up executing before the XR WebGL layer setup has occurred. This was observed in the following symptom:
* a DOM exception would be thrown from inside the renderVR function, which is only called from XR's requestAnimationFrame callback
* following this, the webgl thread would panic with a GL error when executing the GL operations invoked from XRWebGLLayer::Constructor

This indicates that _somehow_ the XR rAF callback that raised an exception would have been operating on GL state that was not actually the state intended.

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components/script/dom/xrwebgllayer.rs

@@ -192,6 +192,9 @@ impl XRWebGLLayer {
             return Err(Error::Operation);
         }
 
+        // Ensure that we finish setting up this layer before continuing.
+        context.Finish();
+
         // Step 10. "Return layer."
         Ok(XRWebGLLayer::new(
             &global.global(),