Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/java/gin_java_bridge_dispatcher.h

Index: content/renderer/java/gin_java_bridge_dispatcher.h
diff --git a/content/renderer/java/gin_java_bridge_dispatcher.h b/content/renderer/java/gin_java_bridge_dispatcher.h
index f795c288f7842ca9cfcc35e84f8f2931e851fad4..297da75247d13cfc36cab08c1a67418ffb4e5a67 100644
--- a/content/renderer/java/gin_java_bridge_dispatcher.h
+++ b/content/renderer/java/gin_java_bridge_dispatcher.h
@@ -59,6 +59,9 @@ class GinJavaBridgeDispatcher
   void OnGinJavaBridgeObjectDeleted(GinJavaBridgeObject* object);
 
  private:
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   void OnAddNamedObject(const std::string& name,
                         ObjectID object_id);
   void OnRemoveNamedObject(const std::string& name);