Add trace event for addition/removal of GPU rasterization trigger

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 5df8fc4ac529125338aad564e79f8ab93e7855aa..63a339893d8dc1aba4ee5b86eb3374cc75b1d8c1 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -619,6 +619,24 @@ bool LayerTreeHost::UseGpuRasterization() const {
   }
 }
 
+void LayerTreeHost::SetHasGpuRasterizationTrigger(bool has_trigger) {
+  if (has_trigger == has_gpu_rasterization_trigger_)
+    return;
+
+  has_gpu_rasterization_trigger_ = has_trigger;
+  if (has_gpu_rasterization_trigger_) {
+    TRACE_EVENT_INSTANT0(

[ernstm, 2014/06/02 21:34:48]

Could you issue a TRACE_EVENT_INSTANT1 with name
"LayerTreeHost::SetHasGpuRasterizationTrigger" and a boolean data item
"has_trigger"? That would make it easier to process these events in telemetry.

[ajuma, 2014/06/03 14:47:11]

Done.

+        "cc",
+        "LayerTreeHost::SetHasGpuRasterizationTrigger trigger added",
+        TRACE_EVENT_SCOPE_THREAD);
+  } else {
+    TRACE_EVENT_INSTANT0(
+        "cc",
+        "LayerTreeHost::SetHasGpuRasterizationTrigger trigger removed",
+        TRACE_EVENT_SCOPE_THREAD);
+  }
+}
+
 void LayerTreeHost::SetViewportSize(const gfx::Size& device_viewport_size) {
   if (device_viewport_size == device_viewport_size_)
     return;