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

cc/tiles/image_controller.h

Index: cc/tiles/image_controller.h
diff --git a/cc/tiles/image_controller.h b/cc/tiles/image_controller.h
index d9253368bb863764161c428597ad765d3f58f712..ac79c82a3f6e5211e2682015beb0b987872a91c7 100644
--- a/cc/tiles/image_controller.h
+++ b/cc/tiles/image_controller.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "base/callback.h"
+#include "base/containers/flat_map.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
@@ -93,7 +94,7 @@ class CC_EXPORT ImageController {
   std::vector<DrawImage> predecode_locked_images_;
 
   static ImageDecodeRequestId s_next_image_decode_queue_id_;
-  std::unordered_map<ImageDecodeRequestId, DrawImage> requested_locked_images_;
+  base::flat_map<ImageDecodeRequestId, DrawImage> requested_locked_images_;
 
   base::SequencedTaskRunner* origin_task_runner_ = nullptr;