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

cc/surfaces/surface_aggregator.h

Index: cc/surfaces/surface_aggregator.h
diff --git a/cc/surfaces/surface_aggregator.h b/cc/surfaces/surface_aggregator.h
index b5a7fb938ff1bc143c084d6409f999b995c79cf7..741d83c13d5cbf8589e20a46964e06f74bde26bc 100644
--- a/cc/surfaces/surface_aggregator.h
+++ b/cc/surfaces/surface_aggregator.h
@@ -36,7 +36,7 @@ class CC_SURFACES_EXPORT SurfaceAggregatorClient {
 
 class CC_SURFACES_EXPORT SurfaceAggregator {
  public:
-  using SurfaceIndexMap = std::unordered_map<SurfaceId, int, SurfaceIdHash>;
+  using SurfaceIndexMap = std::unordered_map<SurfaceId, int, SurfaceId::Hasher>;
 
   SurfaceAggregator(SurfaceAggregatorClient* client,
                     SurfaceManager* manager,
@@ -116,13 +116,13 @@ class CC_SURFACES_EXPORT SurfaceAggregator {
   using RenderPassIdAllocatorMap =
       std::unordered_map<SurfaceId,
                          scoped_ptr<RenderPassIdAllocator>,
-                         SurfaceIdHash>;
+                         SurfaceId::Hasher>;
   RenderPassIdAllocatorMap render_pass_allocator_map_;
   int next_render_pass_id_;
   const bool aggregate_only_damaged_;
 
   using SurfaceToResourceChildIdMap =
-      std::unordered_map<SurfaceId, int, SurfaceIdHash>;
+      std::unordered_map<SurfaceId, int, SurfaceId::Hasher>;
   SurfaceToResourceChildIdMap surface_id_to_resource_child_id_;
 
   // The following state is only valid for the duration of one Aggregate call
@@ -140,7 +140,7 @@ class CC_SURFACES_EXPORT SurfaceAggregator {
   SurfaceIndexMap contained_surfaces_;
 
   // After surface validation, every Surface in this set is valid.
-  std::unordered_set<SurfaceId, SurfaceIdHash> valid_surfaces_;
+  std::unordered_set<SurfaceId, SurfaceId::Hasher> valid_surfaces_;
 
   // This is the pass list for the aggregated frame.
   RenderPassList* dest_pass_list_;