UMA metrics for use count of wheel and touch scrolls.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 1974eb1dcc8fc394ae5a201b8d53a867833d1892..1bf68060e19d0f95e3447b5b1ccac7e0b0ce316a 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2690,6 +2690,8 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBeginImpl(
   client_->RenewTreePriority();
   RecordCompositorSlowScrollMetric(type, CC_THREAD);
 
+  UpdateScrollSourceInfo(wheel_scrolling_);
+
   return scroll_status;
 }
 
@@ -3486,6 +3488,11 @@ std::unique_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
       active_tree_->elastic_overscroll()->PullDeltaForMainThread();
   scroll_info->swap_promises.swap(swap_promises_for_main_thread_scroll_update_);
 
+  // Record and reset scroll source flags.
+  scroll_info->has_scrolled_by_wheel = has_scrolled_by_wheel_;
+  scroll_info->has_scrolled_by_touch = has_scrolled_by_touch_;
+  has_scrolled_by_wheel_ = has_scrolled_by_touch_ = false;
+
   return scroll_info;
 }
 
@@ -4188,4 +4195,11 @@ void LayerTreeHostImpl::SetContextVisibility(bool is_visible) {
   }
 }
 
+void LayerTreeHostImpl::UpdateScrollSourceInfo(bool is_wheel_scroll) {
+  if (is_wheel_scroll)
+    has_scrolled_by_wheel_ = true;
+  else
+    has_scrolled_by_touch_ = true;
+}
+
 }  // namespace cc