Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/devtools/devtools_client.h

 // Copyright (c) 2012 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_RENDERER_DEVTOOLS_DEVTOOLS_CLIENT_H_
 #define CONTENT_RENDERER_DEVTOOLS_DEVTOOLS_CLIENT_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 20 matching lines @@
     : public RenderFrameObserver,
       NON_EXPORTED_BASE(public blink::WebDevToolsFrontendClient) {
  public:
   DevToolsClient(RenderFrame* main_render_frame,
                  const std::string& compatibility_script);
   ~DevToolsClient() override;
 
  private:
   // RenderFrameObserver overrides.
   void DidClearWindowObject() override;
+  void OnDestruct() override;
 
   // WebDevToolsFrontendClient implementation.
   void sendMessageToEmbedder(const blink::WebString&) override;
 
   bool isUnderTest() override;
 
   void OnDispatchOnInspectorFrontend(const std::string& message,
                                      uint32_t total_size);
 
   std::string compatibility_script_;
   std::unique_ptr<blink::WebDevToolsFrontend> web_tools_frontend_;
 
   DISALLOW_COPY_AND_ASSIGN(DevToolsClient);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_DEVTOOLS_DEVTOOLS_CLIENT_H_