Move RenderStats enumeration out of gpu_benchmarking_extension.

cc/rendering_stats.h

Index: cc/rendering_stats.h
diff --git a/cc/rendering_stats.h b/cc/rendering_stats.h
index 995819c645d7d6940bd484fc7c5fc86e935b4be2..4bb4edd2550e829a4ec4ecac3d4ecdeec506fc0e 100644
--- a/cc/rendering_stats.h
+++ b/cc/rendering_stats.h
@@ -24,8 +24,26 @@ struct CC_EXPORT RenderingStats {
     int64 numImplThreadScrolls;
     int64 numMainThreadScrolls;
     int64 numLayersInLayerTree;
+    // Note: when adding new members, please remember to update enumerateFields
+    // in rendering_stats.cc.
 
     RenderingStats();
+
+    // In conjunction with enumerateFields, this allows the embedder to
+    // enumerate the values in this structure without
+    // having to embed references to its specific member variables. This
+    // simplifies the addition of new fields to this type.
+    class Enumerator {
+    public:
+        virtual void addInt64(const char* name, int64 value) = 0;
+        virtual void addDouble(const char* name, double value) = 0;
+
+    protected:
+        virtual ~Enumerator() { }
+    };
+
+    // Outputs the fields in this structure to the provided enumerator.
+    void enumerateFields(Enumerator* enumerator) const;
 };
 
 }  // namespace cc