Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

chrome/renderer/spellchecker/spellcheck_provider.h

 // Copyright (c) 2012 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 CHROME_RENDERER_SPELLCHECKER_SPELLCHECK_PROVIDER_H_
 #define CHROME_RENDERER_SPELLCHECKER_SPELLCHECK_PROVIDER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 53 matching lines @@
 
  private:
   friend class TestingSpellCheckProvider;
 
   // Tries to satisfy a spell check request from the cache in |last_request_|.
   // Returns true (and cancels/finishes the completion) if it can, false
   // if the provider should forward the query on.
   bool SatisfyRequestFromCache(const base::string16& text,
                                blink::WebTextCheckingCompletion* completion);
 
+  // RenderViewObserver implementation.
+  void OnDestruct() override;
+
   // blink::WebSpellCheckClient implementation.
   void spellCheck(
       const blink::WebString& text,
       int& offset,
       int& length,
       blink::WebVector<blink::WebString>* optional_suggestions) override;
   void checkTextOfParagraph(
       const blink::WebString& text,
       blink::WebTextCheckingTypeMask mask,
       blink::WebVector<blink::WebTextCheckingResult>* results) override;
@@ skipping 41 matching lines @@
   // True if the browser is showing the spelling panel for us.
   bool spelling_panel_visible_;
 
   // Weak pointer to shared (per RenderView) spellcheck data.
   SpellCheck* spellcheck_;
 
   DISALLOW_COPY_AND_ASSIGN(SpellCheckProvider);
 };
 
 #endif  // CHROME_RENDERER_SPELLCHECKER_SPELLCHECK_PROVIDER_H_