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 f06a4ad22d78cfef8ab75523c5f5319a1e85ec54..bd39e74dfff848880d372cd2887078313009d771 100644
--- a/cc/trees/tree_synchronizer_unittest.cc
+++ b/cc/trees/tree_synchronizer_unittest.cc
@@ -505,7 +505,7 @@ 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);

[suzyh_UTC10 (ex-contributor), 2017/05/05 04:27:30]

Does the behaviour of this test change at all, given that the second ID value
that you're removing has a non-zero value this time?

[chrishtr, 2017/05/05 05:34:52]

It shouldn't, because cc doesn't understand secondary ids.
Don't know why the 4 was chosen, but from cc'd point of view,
if either primary or secondary key differed, the ElementId was
different.

   scroll_layer->SetElementId(scroll_element_id);
 
   transient_scroll_layer->SetScrollClipLayerId(
@@ -552,7 +552,7 @@ 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);