Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chrome/renderer/chrome_render_frame_observer.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 "chrome/renderer/chrome_render_frame_observer.h"
 
 #include <stddef.h>
 #include <string.h>
 
 #include <limits>
@@ skipping 402 matching lines @@
     case blink::WebMeaningfulLayout::FinishedParsing:
       CapturePageText(PRELIMINARY_CAPTURE);
       break;
     case blink::WebMeaningfulLayout::FinishedLoading:
       CapturePageText(FINAL_CAPTURE);
       break;
     default:
       break;
   }
 }
+
+void ChromeRenderFrameObserver::OnDestruct() {
+  delete this;
+}