Auto merge of #21959 - servo:webgl, r=jdm

Fix a couple of Drop implementations for WebGL objects

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components/script/dom/webglbuffer.rs

@@ -161,6 +161,5 @@ impl WebGLBuffer {
 impl Drop for WebGLBuffer {
     fn drop(&mut self) {
         self.mark_for_deletion();
-        assert!(self.is_deleted());
     }
 }

components/script/dom/webglrenderbuffer.rs

@@ -163,3 +163,9 @@ impl WebGLRenderbuffer {
         Ok(())
     }
 }
+
+impl Drop for WebGLRenderbuffer {
+    fn drop(&mut self) {
+        self.delete();
+    }
+}