[Suggestions Component] Add missing NULL check to ImageManager::OnImageFetched().

components/suggestions/image_fetcher_delegate.h

 // 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.
 
 #ifndef COMPONENTS_SUGGESTIONS_IMAGE_FETCHER_DELEGATE_H_
 #define COMPONENTS_SUGGESTIONS_IMAGE_FETCHER_DELEGATE_H_
 
 class GURL;
 class SkBitmap;
 
 namespace suggestions {
 
 class ImageFetcherDelegate {
  public:
   ImageFetcherDelegate() {}
 
   // Called when an image was fetched. |url| represents the website for which
-  // the image was fetched. |bitmap| is deleted once out of scope.
+  // the image was fetched. |bitmap| stores image data owned by the caller, and
+  // can be nullptr.
   virtual void OnImageFetched(const GURL& url, const SkBitmap* bitmap) = 0;
 
  protected:
   virtual ~ImageFetcherDelegate() {}
 
   DISALLOW_COPY_AND_ASSIGN(ImageFetcherDelegate);
 };
 
 }  // namespace suggestions
 
 #endif  // COMPONENTS_SUGGESTIONS_IMAGE_FETCHER_DELEGATE_H_