Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/image_downloader/image_downloader_impl.cc

 // 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.
 
 #include "content/renderer/image_downloader/image_downloader_impl.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/logging.h"
@@ skipping 204 matching lines @@
 
 void ImageDownloaderImpl::ReplyDownloadResult(
     int32_t http_status_code,
     const std::vector<SkBitmap>& result_images,
     const std::vector<gfx::Size>& result_original_image_sizes,
     const DownloadImageCallback& callback) {
   callback.Run(http_status_code, mojo::Array<SkBitmap>::From(result_images),
                result_original_image_sizes);
 }
 
+void ImageDownloaderImpl::OnDestruct() {
+  delete this;
+}
+
 }  // namespace content