Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

components/plugins/renderer/webview_plugin.cc

 // Copyright 2013 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 "components/plugins/renderer/webview_plugin.h"
 
 #include <stddef.h>
 
 #include "base/auto_reset.h"
 #include "base/message_loop/message_loop.h"
@@ skipping 318 matching lines @@
 
   global->Set(gin::StringToV8(isolate, "plugin"),
               delegate_->GetV8Handle(isolate));
 }
 
 void WebViewPlugin::didReceiveResponse(unsigned identifier,
                                        const WebURLResponse& response) {
   WebFrameClient::didReceiveResponse(identifier, response);
 }
 
-void WebViewPlugin::OnDestruct() {
-  // By default RenderViewObservers are destroyed along with the RenderView.
-  // WebViewPlugin has a custom destruction mechanism, so we disable this.
-}
+void WebViewPlugin::OnDestruct() {}
 
 void WebViewPlugin::OnZoomLevelChanged() {
   if (container_) {
     web_view_->setZoomLevel(
       blink::WebView::zoomFactorToZoomLevel(container_->pageZoomFactor()));
   }
 }