cc: Replace some instances of unordered_map with base::flat_map.

cc/paint/discardable_image_map.h

 // Copyright 2015 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 CC_PAINT_DISCARDABLE_IMAGE_MAP_H_
 #define CC_PAINT_DISCARDABLE_IMAGE_MAP_H_
 
-#include <unordered_map>
 #include <utility>
 #include <vector>
 
+#include "base/containers/flat_map.h"
 #include "cc/base/rtree.h"
 #include "cc/paint/draw_image.h"
 #include "cc/paint/image_id.h"
 #include "cc/paint/paint_export.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkRefCnt.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace cc {
@@ skipping 30 matching lines @@
   gfx::Rect GetRectForImage(ImageId image_id) const;
 
  private:
   friend class ScopedMetadataGenerator;
   friend class DiscardableImageMapTest;
 
   std::unique_ptr<SkCanvas> BeginGeneratingMetadata(const gfx::Size& bounds);
   void EndGeneratingMetadata();
 
   std::vector<std::pair<DrawImage, gfx::Rect>> all_images_;
-  std::unordered_map<ImageId, gfx::Rect> image_id_to_rect_;
+  base::flat_map<ImageId, gfx::Rect> image_id_to_rect_;
 
   RTree images_rtree_;
 };
 
 }  // namespace cc
 
 #endif  // CC_PAINT_DISCARDABLE_IMAGE_MAP_H_