Add a sampling tracing to RenderWidget::DoDeferredUpdate

content/renderer/render_widget.cc

Index: content/renderer/render_widget.cc
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index bb06cfe956eaafc0f7e210d0a5f394ea25c9ea6a..540c3a34ccfc5196f1437675e2ea70810108435c 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -1099,6 +1099,10 @@ void RenderWidget::DoDeferredUpdateAndSendInputAck() {
 
 void RenderWidget::DoDeferredUpdate() {
   TRACE_EVENT0("renderer", "RenderWidget::DoDeferredUpdate");
+// Not supported in split-dll build. http://crbug.com/237249

[nduca, 2013/07/03 05:34:24]

why? the bug you link is the catch all bug. If a trace event feature doesn't
work in split dll, we should have a bug specific to that and have identified if
it will never work, or (hopefully) is just a temporary thing. And if its a
temporary thing, who owns resolving it.

[haraken, 2013/07/03 07:33:15]

Done.

+#if !defined(CHROME_SPLIT_DLL)
+  TraceEventSamplingState0Scope samplingScope("Chrome\0Paint");

[nduca, 2013/07/03 05:34:24]

What is the null character's role in this? How does this get handled?

[haraken, 2013/07/03 07:33:15]

It means "category\0name". This is the normal format of trace events and already
handled in the current implementation.

But I changed the macro so that we can write it as follows:

  TRACE_EVENT_SAMPLING_STATE_SCOPE("Chrome", "Paint");

+#endif
 
   if (!webwidget_)
     return;