Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/image_downloader/image_downloader_impl.h

 // Copyright 2015 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_IMAGE_DOWNLOADER_IMAGE_DOWNLOADER_IMPL_H_
 #define CONTENT_RENDERER_IMAGE_DOWNLOADER_IMAGE_DOWNLOADER_IMPL_H_
 
 #include <stdint.h>
 
 #include <vector>
@@ skipping 27 matching lines @@
 
   // RenderThreadObserver implementation.
   void OnRenderProcessShutdown() override;
 
  private:
   ImageDownloaderImpl(
       RenderFrame* render_frame,
       mojo::InterfaceRequest<content::mojom::ImageDownloader> request);
   ~ImageDownloaderImpl() override;
 
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   // ImageDownloader methods:
   void DownloadImage(const mojo::String& url,
                      bool is_favicon,
                      uint32_t max_bitmap_size,
                      bool bypass_cache,
                      const DownloadImageCallback& callback) override;
 
   // Requests to fetch an image. When done, the ImageDownloaderImpl
   // is notified by way of DidFetchImage. Returns true if the
   // request was successfully started, false otherwise.
@@ skipping 32 matching lines @@
 
   // ImageResourceFetchers schedule via FetchImage.
   ImageResourceFetcherList image_fetchers_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageDownloaderImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_IMAGE_DOWNLOADER_IMAGE_DOWNLOADER_IMPL_H_