Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/mojo_bindings_controller.h

Index: content/renderer/mojo_bindings_controller.h
diff --git a/content/renderer/mojo_bindings_controller.h b/content/renderer/mojo_bindings_controller.h
index d26f17a85cf13dcaf546a142696a0c1cfb2fe007..b0a3fb6f8e8c9d0cf310e5655bdc4ad85a0f0f94 100644
--- a/content/renderer/mojo_bindings_controller.h
+++ b/content/renderer/mojo_bindings_controller.h
@@ -43,6 +43,7 @@ class MojoBindingsController
   void WillReleaseScriptContext(v8::Local<v8::Context> context,
                                 int world_id) override;
   void DidClearWindowObject() override;
+  void OnDestruct() override;
 
   const bool for_layout_tests_;