Record size of scroller that user 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 f5d61590fc92298ce666ef4bc98d379c68f9f6d4..bd772e74f89b320240a4fb601bc68ed7239174f8 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -168,6 +168,10 @@ LayerTreeHostImpl::FrameData::FrameData()
 
 LayerTreeHostImpl::FrameData::~FrameData() {}
 
+std::pair<const int, const int>
+    LayerTreeHostImpl::m_pairForScrollerSizeHistograms =
+        std::make_pair(200000, 50);

[tdresser, 2017/03/30 14:41:35]

Name this so it's clearer what this contains. What's the first thing? What's the
second?

I think this will probably be clearer if we split them out of the pair, and give
them both meaningful names.

+
 std::unique_ptr<LayerTreeHostImpl> LayerTreeHostImpl::Create(
     const LayerTreeSettings& settings,
     LayerTreeHostImplClient* client,
@@ -2751,6 +2755,21 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     scrolling_node =
         scrolling_layer ? scroll_tree.Node(scrolling_layer->scroll_tree_index())
                         : nullptr;
+    if (!scroll_on_main_thread && scrolling_node) {
+      if (IsWheelBasedScroll(type)) {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Wheel",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            m_pairForScrollerSizeHistograms.first,
+            m_pairForScrollerSizeHistograms.second);
+      } else {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Touch",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            m_pairForScrollerSizeHistograms.first,
+            m_pairForScrollerSizeHistograms.second);
+      }
+    }
   }
 
   if (scroll_on_main_thread) {