Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

android_webview/renderer/print_render_frame_observer.h

 // Copyright 2014 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 ANDROID_WEBVIEW_RENDERER_PRINT_RENDER_FRAME_OBSERVER_H_
 #define ANDROID_WEBVIEW_RENDERER_PRINT_RENDER_FRAME_OBSERVER_H_
 
 #include "base/macros.h"
 #include "content/public/renderer/render_frame_observer.h"
 
 namespace android_webview {
 
 class PrintRenderFrameObserver : public content::RenderFrameObserver {
  public:
   explicit PrintRenderFrameObserver(content::RenderFrame* render_view);
 
  private:
   ~PrintRenderFrameObserver() override;
 
   // RenderFrameObserver implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
+  void OnDestruct() override;
 
   // IPC handlers
   void OnPrintNodeUnderContextMenu();
 
   DISALLOW_COPY_AND_ASSIGN(PrintRenderFrameObserver);
 };
 
 }  // namespace android_webview
 
 #endif  // ANDROID_WEBVIEW_RENDERER_PRINT_RENDER_FRAME_OBSERVER_H_