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 c3005c8a4bfaa15b1db877b8618f0f341f2d64fc..b50177aca1d8de6823a72c5efe8121f3d3cbfbe7 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -36,6 +36,7 @@
 #include "cc/input/scroll_elasticity_helper.h"
 #include "cc/input/scroll_state.h"
 #include "cc/input/scrollbar_animation_controller.h"
+#include "cc/input/scroller_size.h"
 #include "cc/layers/append_quads_data.h"
 #include "cc/layers/effect_tree_layer_list_iterator.h"
 #include "cc/layers/heads_up_display_layer_impl.h"
@@ -2768,6 +2769,24 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     scrolling_node = FindScrollNodeForDeviceViewportPoint(
         device_viewport_point, type, layer_impl, &scroll_on_main_thread,
         &scroll_status.main_thread_scrolling_reasons);
+    if (!scroll_on_main_thread && scrolling_node &&
+        InnerViewportScrollLayer() && OuterViewportScrollLayer() &&

[flackr, 2017/04/20 07:15:01]

Why do we need to have a inner and outer viewport scroll layer to record
overflow scroller sizes?

[bokan, 2017/04/20 14:34:06]

I think it's meant to null check the conditions below but this would be clearer
and more correct like this:

(!InnerViewportScrollLayer() || scrolling_node->element_id !=
InnerViewportScrollLayer()->element_id())

That said, I *think* this check is equivalent to
!scrolling_node->scrolls_inner_viewport

+        scrolling_node->element_id !=
+            InnerViewportScrollLayer()->element_id() &&
+        scrolling_node->element_id !=
+            OuterViewportScrollLayer()->element_id()) {
+      if (IsWheelBasedScroll(type)) {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Wheel",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            kMaxScrollerSize, kScrollerSizeBucketCount);
+      } else {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Touch",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            kMaxScrollerSize, kScrollerSizeBucketCount);
+      }
+    }
   }
 
   if (scroll_on_main_thread) {