Use flat_set for ResourceIdSet

cc/base/resource_id.h

Index: cc/base/resource_id.h
diff --git a/cc/base/resource_id.h b/cc/base/resource_id.h
index 2b25ea55feefe05e8da321f23b8ba1066180f6aa..5ae6189cfe7e61ff4ffa3cfc5e6cfd3fd92474c6 100644
--- a/cc/base/resource_id.h
+++ b/cc/base/resource_id.h
@@ -6,12 +6,13 @@
 #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