Add impl/comp scrolling instrumentation.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index aa4a002dff41294c5799627a77fccf72889a4437..23419e83a8bd3e67f05f425ea2e3295dcbcc5395 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -6,6 +6,7 @@
 
 #include <algorithm>
 
+#include "base/debug/trace_event.h"
 #include "base/location.h"
 #include "base/metrics/histogram.h"
 #include "base/single_thread_task_runner.h"
@@ -563,6 +564,10 @@ void Layer::SetScrollOffset(gfx::Vector2d scroll_offset) {
   DCHECK(IsPropertyChangeAllowed());
   if (scroll_offset_ == scroll_offset)
     return;
+  TRACE_EVENT2("impl-scroll",
+               "Layer::SetScrollOffset",
+               "scroll_x", scroll_offset.x(),
+               "scroll_y", scroll_offset.y());
   scroll_offset_ = scroll_offset;
   SetNeedsCommit();
 }
@@ -574,6 +579,10 @@ void Layer::SetScrollOffsetFromImplSide(gfx::Vector2d scroll_offset) {
   DCHECK(layer_tree_host_ && layer_tree_host_->CommitRequested());
   if (scroll_offset_ == scroll_offset)
     return;
+  TRACE_EVENT2("impl-scroll",
+               "Layer::SetScrollOffsetFromImplSide",
+               "scroll_x", scroll_offset.x(),
+               "scroll_y", scroll_offset.y());
   scroll_offset_ = scroll_offset;
   SetNeedsPushProperties();
   if (!did_scroll_callback_.is_null())