cc: Add a checkerboard trace event instant to picture layer impl.

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index bd3b9dc590e413bf998aa6325f95d8fd36386239..394ec72d247d72ba79f88b3c82dbfbf60676040a 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -251,6 +251,10 @@ void PictureLayerImpl::AppendQuads(QuadSink* quad_sink,
         visible_geometry_rect.width() * visible_geometry_rect.height();
 
     if (!*iter || !iter->IsReadyToDraw()) {
+      TRACE_EVENT_INSTANT0("cc",

[danakj, 2014/05/09 19:20:14]

This could potentially happen an awful lot during a scroll. Wanna maybe do this
at most once per call to AppendDrawQuads?

[vmpstr, 2014/05/09 19:29:04]

Makes sense. Done.

+                           "PictureLayerImpl::AppendQuads checkerboard",
+                           TRACE_EVENT_SCOPE_THREAD);
+
       if (draw_checkerboard_for_missing_tiles()) {
         scoped_ptr<CheckerboardDrawQuad> quad = CheckerboardDrawQuad::Create();
         SkColor color = DebugColors::DefaultCheckerboardColor();