Change cc::ElementId to be a uint64_t

cc/trees/tree_synchronizer_unittest.cc

Index: cc/trees/tree_synchronizer_unittest.cc
diff --git a/cc/trees/tree_synchronizer_unittest.cc b/cc/trees/tree_synchronizer_unittest.cc
index ad13e8ddf435b8bf54db9be1e4cadf4f0b4e72b2..1def71a2b6774995c0633d53b960607fa8fad8cb 100644
--- a/cc/trees/tree_synchronizer_unittest.cc
+++ b/cc/trees/tree_synchronizer_unittest.cc
@@ -504,7 +504,7 @@ TEST_F(TreeSynchronizerTest, SynchronizeCurrentlyScrollingNode) {
   transient_scroll_layer->AddChild(scroll_clip_layer);
   scroll_clip_layer->AddChild(scroll_layer);
 
-  ElementId scroll_element_id = ElementId(5, 4);
+  ElementId scroll_element_id = ElementId(5);
   scroll_layer->SetElementId(scroll_element_id);
 
   transient_scroll_layer->SetScrollClipLayerId(
@@ -551,7 +551,7 @@ TEST_F(TreeSynchronizerTest, SynchronizeScrollTreeScrollOffsetMap) {
   scoped_refptr<Layer> transient_scroll_clip_layer = Layer::Create();
   scoped_refptr<Layer> transient_scroll_layer = Layer::Create();
 
-  ElementId scroll_element_id = ElementId(5, 4);
+  ElementId scroll_element_id = ElementId(5);
   scroll_layer->SetElementId(scroll_element_id);
 
   layer_tree_root->AddChild(transient_scroll_clip_layer);