Add TRACE_EVENT_IS_NEW_TRACE as a way to snapshot objects at start of recording

cc/resources/picture.cc

Index: cc/resources/picture.cc
diff --git a/cc/resources/picture.cc b/cc/resources/picture.cc
index b0f3fa30e59f0587759f402efd64fd6fe71392da..2f55eb377700cd470fa48e76077fdf9da9f5f361 100644
--- a/cc/resources/picture.cc
+++ b/cc/resources/picture.cc
@@ -226,7 +226,7 @@ void Picture::CloneForDrawing(int num_threads) {
     clones_.push_back(clone);
 
     TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
-      "cc::Picture", clone.get(), TracedPicture::AsTraceablePicture(clone));
+        "cc::Picture", clone.get(), TracedPicture::AsTraceablePicture(clone));
   }
 }
 
@@ -273,7 +273,7 @@ void Picture::Record(ContentLayerClient* painter,
 
   opaque_rect_ = gfx::ToEnclosedRect(opaque_layer_rect);
 
-    TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
+  TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(TRACE_DISABLED_BY_DEFAULT("cc.debug"),
       "cc::Picture", this, TracedPicture::AsTraceablePicture(this));
 }
 
@@ -388,6 +388,11 @@ scoped_ptr<Value> Picture::AsValue() const {
   return res.PassAs<base::Value>();
 }
 
+void Picture::DidBeginTracing() {
+  TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(TRACE_DISABLED_BY_DEFAULT("cc.debug"),

[enne (OOO), 2013/05/31 23:27:21]

What about making this Picture::EmitTraceSnapshot or something and calling it
from CloneForDrawing too so that it's not repeated?

+      "cc::Picture", this, TracedPicture::AsTraceablePicture(this));
+}
+
 base::LazyInstance<Picture::PixelRefs>
     Picture::PixelRefIterator::empty_pixel_refs_;