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

cc/surfaces/surface_id_allocator.cc

Index: cc/surfaces/surface_id_allocator.cc
diff --git a/cc/surfaces/surface_id_allocator.cc b/cc/surfaces/surface_id_allocator.cc
index 04f60d9fad7fcf6b7688e66ea0309f783fa8eb8a..430f01d99e61f993ffd2881f8dcffaf622099972 100644
--- a/cc/surfaces/surface_id_allocator.cc
+++ b/cc/surfaces/surface_id_allocator.cc
@@ -24,14 +24,15 @@ SurfaceIdAllocator::~SurfaceIdAllocator() {
 }
 
 SurfaceId SurfaceIdAllocator::GenerateId() {
-  SurfaceId id(static_cast<uint64_t>(id_namespace_) << 32 | next_id_);
+  SurfaceId id = SurfaceId::FromUnsafeValue(
+      static_cast<uint64_t>(id_namespace_) << 32 | next_id_);
   next_id_++;
   return id;
 }
 
 // static
 uint32_t SurfaceIdAllocator::NamespaceForId(SurfaceId id) {
-  return id.id >> 32;
+  return id.GetUnsafeValue() >> 32;
 }
 
 }  // namespace cc