Reusing base::IdType<...> to implement SurfaceId.

cc/surfaces/surface_id.h

Index: cc/surfaces/surface_id.h
diff --git a/cc/surfaces/surface_id.h b/cc/surfaces/surface_id.h
index c0c4410d973f1512ce0268e5d0dd30429dc22edd..d257d02322de41bb7ffeb93ec09b2566daaac455 100644
--- a/cc/surfaces/surface_id.h
+++ b/cc/surfaces/surface_id.h
@@ -5,39 +5,12 @@
 #ifndef CC_SURFACES_SURFACE_ID_H_
 #define CC_SURFACES_SURFACE_ID_H_
 
-#include <stddef.h>
-#include <stdint.h>
-
-#include <functional>
+#include "base/id_type.h"
 
 namespace cc {
 
-struct SurfaceId {
-  SurfaceId() : id(0) {}
-  explicit SurfaceId(uint64_t id) : id(id) {}
-
-  bool is_null() const { return id == 0; }
-
-  uint64_t id;
-};
-
-inline bool operator==(const SurfaceId& a, const SurfaceId& b) {
-  return a.id == b.id;
-}
-
-inline bool operator!=(const SurfaceId& a, const SurfaceId& b) {
-  return !(a == b);
-}
-
-inline bool operator<(const SurfaceId& a, const SurfaceId& b) {
-  return a.id < b.id;
-}
-
-struct SurfaceIdHash {
-  size_t operator()(const SurfaceId& key) const {
-    return std::hash<uint64_t>()(key.id);
-  }
-};
+class Surface;
+using SurfaceId = base::IdTypeU64<Surface>;
 
 }  // namespace cc