Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/manifest/manifest_manager.cc

 // 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.
 
 #include "content/renderer/manifest/manifest_manager.h"
 
 #include "base/bind.h"
 #include "base/strings/nullable_string16.h"
 #include "content/common/manifest_manager_messages.h"
 #include "content/public/renderer/render_frame.h"
@@ skipping 188 matching lines @@
 
   std::list<GetManifestCallback> callbacks;
   callbacks.swap(pending_callbacks_);
 
   for (std::list<GetManifestCallback>::const_iterator it = callbacks.begin();
        it != callbacks.end(); ++it) {
     it->Run(manifest_, manifest_debug_info_);
   }
 }
 
+void ManifestManager::OnDestruct() {
+  delete this;
+}
+
 } // namespace content