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..992d61d3e0481445f0f0ba9f5402c005188f0a64 100644
--- a/cc/trees/element_id.h
+++ b/cc/trees/element_id.h
@@ -5,74 +5,13 @@
 #ifndef CC_TREES_ELEMENT_ID_H_
 #define CC_TREES_ELEMENT_ID_H_
 
-#include <stddef.h>
-
-#include <cstdint>
-#include <functional>
-#include <iosfwd>
-#include <memory>
-
-#include "base/hash.h"
-#include "cc/cc_export.h"
-
-namespace base {
-class Value;
-namespace trace_event {
-class TracedValue;
-}  // namespace trace_event
-}  // namespace base
+#include <stdint.h>
 
 namespace cc {
 
-// Element ids are chosen by cc's clients and can be used as a stable identifier
-// across updates.
-//
-// Historically, the layer tree stored all compositing data but this has been
-// refactored over time into auxilliary structures such as property trees.
-//
-// In composited scrolling, Layers directly reference scroll tree nodes
-// (Layer::scroll_tree_index) but scroll tree nodes are being refactored to
-// reference stable element ids instead of layers. Scroll property nodes have
-// unique element ids that blink creates from scrollable areas (though this is
-// opaque to the compositor). This refactoring of scroll nodes keeping a
-// scrolling element id instead of a scrolling layer id allows for more general
-// compositing where, for example, multiple layers scroll with one scroll node.
-//
-// The animation system (see: ElementAnimations and blink::ElementAnimations) is
-// another auxilliary structure to the layer tree and uses element ids as a
-// stable identifier for animation targets. A Layer's element id can change over
-// 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) {}
-
-  bool operator==(const ElementId& o) const;
-  bool operator!=(const ElementId& o) const;
-  bool operator<(const ElementId& o) const;
-
-  // An ElementId's conversion to a boolean value depends only on its primaryId.
-  explicit operator bool() const;
-
-  void AddToTracedValue(base::trace_event::TracedValue* res) const;
-  std::unique_ptr<base::Value> AsValue() const;
-
-  // 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;
-};
-
-CC_EXPORT ElementId LayerIdToElementIdForTesting(int layer_id);
-
-struct CC_EXPORT ElementIdHash {
-  size_t operator()(ElementId key) const;
-};
+using ElementId = uint64_t;
 
-// Stream operator so ElementId can be used in assertion statements.
-CC_EXPORT std::ostream& operator<<(std::ostream& out, const ElementId& id);
+static const ElementId kInvalidElementId = 0;
 
 }  // namespace cc