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 6edff37f91e42c40b8a3471c3ef07e462840144b..ebd8d171deb4931d9a2aeb6a1cc073abe6616fbe 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -237,7 +237,9 @@ LayerTreeHostImpl::LayerTreeHostImpl(
       requires_high_res_to_draw_(false),
       is_likely_to_require_a_draw_(false),
       has_valid_compositor_frame_sink_(false),
-      mutator_(nullptr) {
+      mutator_(nullptr),
+      has_scrolled_by_wheel_(false),
+      has_scrolled_by_touch_(false) {
   DCHECK(mutator_host_);
   mutator_host_->SetMutatorHostClient(this);
 
@@ -2695,6 +2697,8 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBeginImpl(
   client_->RenewTreePriority();
   RecordCompositorSlowScrollMetric(type, CC_THREAD);
 
+  UpdateScrollSourceInfo(wheel_scrolling_);
+
   return scroll_status;
 }
 
@@ -3515,6 +3519,11 @@ 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;
 }
 
@@ -4217,4 +4226,11 @@ 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