Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/shell/renderer/layout_test/layout_test_render_frame_observer.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_SHELL_RENDERER_LAYOUT_TEST_LAYOUT_TEST_RENDER_FRAME_OBSERVER_H_
 #define CONTENT_SHELL_RENDERER_LAYOUT_TEST_LAYOUT_TEST_RENDER_FRAME_OBSERVER_H_
 
 #include "base/macros.h"
 #include "content/public/renderer/render_frame_observer.h"
 
 namespace IPC {
 class Message;
 }  // namespace IPC
 
 namespace base {
 class DictionaryValue;
 }  // namespace
 
 namespace content {
 struct ShellTestConfiguration;
 class RenderFrame;
 struct ShellTestConfiguration;
 
 class LayoutTestRenderFrameObserver : public RenderFrameObserver {
  public:
   explicit LayoutTestRenderFrameObserver(RenderFrame* render_frame);
   ~LayoutTestRenderFrameObserver() override {}
 
+  // RenderFrameObserver implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
 
  private:
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   void OnLayoutDumpRequest();
   void OnSetTestConfiguration(const ShellTestConfiguration& test_config);
   void OnReplicateTestConfiguration(const ShellTestConfiguration& test_config);
   void OnSetupSecondaryRenderer();
 
   DISALLOW_COPY_AND_ASSIGN(LayoutTestRenderFrameObserver);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_SHELL_RENDERER_LAYOUT_TEST_LAYOUT_TEST_RENDER_FRAME_OBSERVER_H_