Add impl/comp scrolling instrumentation.

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 39cdceb91420f9d1040c3f7e542a7c1fddd1c775..7a7261490ef0dd5932953c00fd579fe220f33f20 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -667,6 +667,14 @@ void ThreadProxy::ScheduledActionSendBeginFrameToMainThread() {
   begin_frame_state->scroll_info =
       layer_tree_host_impl_->ProcessScrollDeltas();
 
+  if (!begin_frame_state->scroll_info->scrolls.empty()) {
+    TRACE_EVENT_INSTANT0(
+        "impl-scroll",
+        "ThreadProxy::ScheduledActionSendBeginFrameToMainThread received scroll"
+        " deltas",
+        TRACE_EVENT_SCOPE_THREAD);
+  }
+
   if (!layer_tree_host_impl_->settings().impl_side_painting) {
     DCHECK_GT(layer_tree_host_impl_->memory_allocation_limit_bytes(), 0u);
   }
@@ -726,8 +734,24 @@ void ThreadProxy::BeginFrameOnMainThread(
     return;
   }
 
-  if (begin_frame_state)
+  if (begin_frame_state) {
+    const bool has_scrolls = !begin_frame_state->scroll_info->scrolls.empty();
+    if (has_scrolls) {
+      // We use async events here because we want to conditionally trace. This
+      // is too spammy otherwise.
+      TRACE_EVENT_ASYNC_BEGIN0(
+          "impl-scroll",
+          "ThreadProxy::BeginFrameOnMainThread",
+          this);
+    }
     layer_tree_host_->ApplyScrollAndScale(*begin_frame_state->scroll_info);
+    if (has_scrolls) {
+      TRACE_EVENT_ASYNC_END0(
+          "impl-scroll",
+          "ThreadProxy::BeginFrameOnMainThread",
+          this);
+    }
+  }
 
   layer_tree_host_->WillBeginFrame();