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..8539907d5fb35a7361802aa5a488ca50c274c476 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2751,6 +2751,17 @@ 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, 90000, 50);

[flackr, 2017/03/28 14:40:23]

Use a pair of constants for the 90000 and 50 and share with ScrollManager.cpp so
that we don't accidentally end up reporting with different buckets.

[yigu, 2017/03/29 15:32:03]

Done.

+      } else {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Touch",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1, 90000, 50);
+      }
+    }
   }
 
   if (scroll_on_main_thread) {