Add builders for tracing event's structural arguments

cc/resources/image_raster_worker_pool.h

Index: cc/resources/image_raster_worker_pool.h
diff --git a/cc/resources/image_raster_worker_pool.h b/cc/resources/image_raster_worker_pool.h
index 2e11a220f41da1ace4568978890687f974ea61f1..39ac7237266ced9c3a7c2c47d0ab31f1467abbd9 100644
--- a/cc/resources/image_raster_worker_pool.h
+++ b/cc/resources/image_raster_worker_pool.h
@@ -10,6 +10,12 @@
 #include "cc/resources/raster_worker_pool.h"
 #include "cc/resources/rasterizer.h"
 
+namespace base {
+namespace debug {
+class ConvertableToTraceFormat;
+}
+}
+
 namespace cc {
 class ResourceProvider;
 
@@ -45,7 +51,7 @@ class CC_EXPORT ImageRasterWorkerPool : public RasterWorkerPool,
  private:
   void OnRasterFinished();
   void OnRasterRequiredForActivationFinished();
-  scoped_ptr<base::Value> StateAsValue() const;
+  scoped_refptr<base::debug::ConvertableToTraceFormat> StateAsValue() const;
 
   scoped_refptr<base::SequencedTaskRunner> task_runner_;
   TaskGraphRunner* task_graph_runner_;