Change cc::ElementId to be a uint64_t

cc/trees/element_id.h

Index: cc/trees/element_id.h
diff --git a/cc/trees/element_id.h b/cc/trees/element_id.h
index 8518cf8ce13393c93c8d01faf224f27620d5269c..059cf2b36d7e8bdcaec6e9372004319f894bcacc 100644
--- a/cc/trees/element_id.h
+++ b/cc/trees/element_id.h
@@ -24,6 +24,10 @@ class TracedValue;
 
 namespace cc {
 
+using ElementIdType = uint64_t;
+
+static const ElementIdType kInvalidElementId = 0;
+
 // Element ids are chosen by cc's clients and can be used as a stable identifier
 // across updates.
 //
@@ -44,9 +48,8 @@ namespace cc {
 // the Layer's lifetime because non-default ElementIds are only set during an
 // animation's lifetime.
 struct CC_EXPORT ElementId {
-  ElementId(int primaryId, int secondaryId)
-      : primaryId(primaryId), secondaryId(secondaryId) {}
-  ElementId() : ElementId(0, 0) {}
+  explicit ElementId(int id) : id_(id) {}
+  ElementId() : ElementId(kInvalidElementId) {}
 
   bool operator==(const ElementId& o) const;
   bool operator!=(const ElementId& o) const;
@@ -61,11 +64,10 @@ struct CC_EXPORT ElementId {
   // The compositor treats this as an opaque handle and should not know how to
   // interpret these bits. Non-blink cc clients typically operate in terms of
   // layers and may set this value to match the client's layer id.
-  int primaryId;
-  int secondaryId;
+  ElementIdType id_;

[wkorman, 2017/05/08 22:25:26]

For the record: we discussed in person re: making this a class rather than a
struct and having this be private. Could pursue subsequently. Only a couple of
areas in Blink read this directly currently (to output as string, and to
retrieve id we know we stuck in in the first place to pull off sub id).

Alternatives discussed trade off data hiding improvements at cost of more code
complexity or performance overhead or both. So leaving as it is seems ok for
now.

To some extent the '_' suffix should make it clear that one shouldn't monkey
with this unless one knows what one is doing.

 };
 
-CC_EXPORT ElementId LayerIdToElementIdForTesting(int layer_id);
+ElementId CC_EXPORT LayerIdToElementIdForTesting(int layer_id);
 
 struct CC_EXPORT ElementIdHash {
   size_t operator()(ElementId key) const;