Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

android_webview/renderer/aw_render_frame_ext.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_AW_RENDER_FRAME_EXT_H_
 #define ANDROID_WEBVIEW_RENDERER_AW_RENDER_FRAME_EXT_H_
 
 #include "base/macros.h"
 #include "content/public/renderer/render_frame_observer.h"
 #include "ui/gfx/geometry/point_f.h"
@@ skipping 18 matching lines @@
 
  private:
   ~AwRenderFrameExt() override;
 
   // RenderFrameObserver:
   void DidCommitProvisionalLoad(bool is_new_navigation,
                                 bool is_same_page_navigation) override;
 
   bool OnMessageReceived(const IPC::Message& message) override;
   void FocusedNodeChanged(const blink::WebNode& node) override;
+  void OnDestruct() override;
 
   void OnDocumentHasImagesRequest(uint32_t id);
   void OnDoHitTest(const gfx::PointF& touch_center,
                    const gfx::SizeF& touch_area);
 
   void OnSetTextZoomFactor(float zoom_factor);
 
   void OnResetScrollAndScaleState();
 
   void OnSetInitialPageScale(double page_scale_factor);
 
   void OnSetBackgroundColor(SkColor c);
 
   void OnSmoothScroll(int target_x, int target_y, int duration_ms);
 
   blink::WebView* GetWebView();
   blink::WebFrameWidget* GetWebFrameWidget();
 
   url::Origin last_origin_;
 
   DISALLOW_COPY_AND_ASSIGN(AwRenderFrameExt);
 };
 
 }
 
 #endif  // ANDROID_WEBVIEW_RENDERER_AW_RENDER_FRAME_EXT_H_