Change cc::ElementId to be a uint64_t

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 6b53b46c2d93cc0762504f59fb46855a80443b63..0dedc808d238a1ec7b19874b1c025dc297e2fc61 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -823,9 +823,8 @@ class CC_EXPORT LayerTreeHostImpl
 
   // Map from scroll element ID to scrollbar animation controller.
   // There is one animation controller per pair of overlay scrollbars.
-  std::unordered_map<ElementId,
-                     std::unique_ptr<ScrollbarAnimationController>,
-                     ElementIdHash>
+  std::unordered_map<ElementId::Id,
+                     std::unique_ptr<ScrollbarAnimationController>>
       scrollbar_animation_controllers_;
 
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;