Use flat_set for ResourceIdSet

cc/base/resource_id.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_BASE_RESOURCE_ID_H_
 #define CC_BASE_RESOURCE_ID_H_
 
 #include <stdint.h>
-#include <unordered_set>
+
+#include "base/containers/flat_set.h"
 
 namespace cc {
 
 using ResourceId = uint32_t;
-using ResourceIdSet = std::unordered_set<ResourceId>;
+using ResourceIdSet = base::flat_set<ResourceId>;
 
 }  // namespace cc
 
 #endif  // CC_BASE_RESOURCE_ID_H_