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 fc9f2daba445f1b78b01dc5a3c2624a620663b95..6af0ba13cfc395e28016eb2df99b84324a5c6173 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"
@@ -557,6 +558,10 @@ void Layer::SetScrollOffset(gfx::Vector2d scroll_offset) {
   DCHECK(IsPropertyChangeAllowed());
   if (scroll_offset_ == scroll_offset)
     return;
+  TRACE_EVENT2("impl-scroll",
+               "Layer::SetScrollOffset",
+               "scrollX", scroll_offset.x(),
+               "scrollY", scroll_offset.y());
   scroll_offset_ = scroll_offset;
   SetNeedsCommit();
 }
@@ -566,6 +571,10 @@ 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",
+               "scrollX", scroll_offset.x(),
+               "scrollY", scroll_offset.y());
   scroll_offset_ = scroll_offset;
   if (!did_scroll_callback_.is_null())
     did_scroll_callback_.Run();