Input latency performance test that uses tracing.

content/browser/renderer_host/render_widget_host.cc

Index: content/browser/renderer_host/render_widget_host.cc
diff --git a/content/browser/renderer_host/render_widget_host.cc b/content/browser/renderer_host/render_widget_host.cc
index d48963e8cd7f42533d0e37695c85ae3f7fded897..4f3ef70abef9d8ebf658752b8bf01a0dae5e86cd 100644
--- a/content/browser/renderer_host/render_widget_host.cc
+++ b/content/browser/renderer_host/render_widget_host.cc
@@ -1036,6 +1036,12 @@ void RenderWidgetHost::OnMsgUpdateRect(
             UserMetricsAction("BadMessageTerminate_RWH1"));
         process()->ReceivedBadMessage();
       } else {
+        UNSHIPPED_TRACE_EVENT_INSTANT2("test_latency", "UpdateRect",

[nduca, 2011/12/21 20:24:23]

is there any risk of the first byte of the dib being null?

You should probably update the JS to explain that its very important the 0,0'th
pixel has a specific result. It was totally not clear to me that this was the
case, when i was reading it.

[jbates, 2011/12/22 02:41:17]

This just accesses the first 32bit value of the dib, so it is ok if it's zero.
Done.

+            "x+y", params.bitmap_rect.x() + params.bitmap_rect.y(),
+            "color", 0xffffff & *static_cast<uint32*>(dib->memory()));
+        UNSHIPPED_TRACE_EVENT_INSTANT1("test_latency", "UpdateRectWidth",
+            "width", params.bitmap_rect.width());
+
         // Scroll the backing store.
         if (!params.scroll_rect.IsEmpty()) {
           ScrollBackingStoreRect(params.dx, params.dy,