Change cc::ElementId to be a uint64_t

cc/trees/element_id.cc

Index: cc/trees/element_id.cc
diff --git a/cc/trees/element_id.cc b/cc/trees/element_id.cc
index 45913303c1019dfa9c36113d6a11fffae12c1bfb..6e32b660aa53b4b69acc5371aa27441193b809a0 100644
--- a/cc/trees/element_id.cc
+++ b/cc/trees/element_id.cc
@@ -13,7 +13,7 @@
 namespace cc {
 
 bool ElementId::operator==(const ElementId& o) const {
-  return primaryId == o.primaryId && secondaryId == o.secondaryId;
+  return id_ == o.id_;
 }
 
 bool ElementId::operator!=(const ElementId& o) const {
@@ -21,36 +21,33 @@ bool ElementId::operator!=(const ElementId& o) const {
 }
 
 bool ElementId::operator<(const ElementId& o) const {
-  return std::tie(primaryId, secondaryId) <
-         std::tie(o.primaryId, o.secondaryId);
+  return id_ < o.id_;
 }
 
 ElementId::operator bool() const {
-  return !!primaryId;
+  return !!id_;
 }
 
 ElementId LayerIdToElementIdForTesting(int layer_id) {
-  return ElementId(std::numeric_limits<int>::max() - layer_id, 0);
+  return ElementId(std::numeric_limits<int>::max() - layer_id);
 }
 
 void ElementId::AddToTracedValue(base::trace_event::TracedValue* res) const {
-  res->SetInteger("primaryId", primaryId);
-  res->SetInteger("secondaryId", secondaryId);
+  res->SetInteger("id_", id_);
 }
 
 std::unique_ptr<base::Value> ElementId::AsValue() const {
   std::unique_ptr<base::DictionaryValue> res(new base::DictionaryValue());
-  res->SetInteger("primaryId", primaryId);
-  res->SetInteger("secondaryId", secondaryId);
+  res->SetInteger("id_", id_);
   return std::move(res);
 }
 
 size_t ElementIdHash::operator()(ElementId key) const {
-  return base::HashInts(key.primaryId, key.secondaryId);
+  return std::hash<int>()(key.id_);
 }
 
 std::ostream& operator<<(std::ostream& out, const ElementId& id) {
-  return out << "(" << id.primaryId << ", " << id.secondaryId << ")";
+  return out << "(" << id.id_ << ")";
 }
 
 }  // namespace cc