Fix use-after-free in gfx::Image.

ui/gfx/image/image.cc

 // 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.
 
 #include "ui/gfx/image/image.h"
 
 #include <algorithm>
 #include <set>
 #include <utility>
 
@@ skipping 734 matching lines @@
   if (it == storage_->representations().end()) {
     CHECK(!must_exist);
     return NULL;
   }
   return it->second.get();
 }
 
 void Image::AddRepresentation(scoped_ptr<internal::ImageRep> rep) const {
   CHECK(storage_.get());
   RepresentationType type = rep->type();
-  storage_->representations().insert(std::make_pair(type, std::move(rep)));
+  auto result =
+      storage_->representations().insert(std::make_pair(type, std::move(rep)));
+
+  // insert should not fail (implies that there was already a representation of
+  // that type in the map).
+  CHECK(result.second) << "type was already in map.";
 }
 
 }  // namespace gfx