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 5bbe7ff77f55811d7157b9976efa6db06ebf2cf5..8cd8c7cb46456c00b44da218d033d903e32f0cee 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/heads_up_display_layer_impl.h"
 #include "cc/layers/layer_impl.h"
@@ -2762,6 +2763,19 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     scrolling_node =
         scrolling_layer ? scroll_tree.Node(scrolling_layer->scroll_tree_index())
                         : nullptr;
+    if (!scroll_on_main_thread && scrolling_node) {

[bokan, 2017/04/04 17:32:57]

if you're going to avoid recording the viewport (aka root layer) on the main
thread as mentioned in ScrollManager, then you'll have to avoid recording it
here as well.

+      if (IsWheelBasedScroll(type)) {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Wheel",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            kMaxScrollerSize, kBucketNum);
+      } else {
+        UMA_HISTOGRAM_CUSTOM_COUNTS(
+            "Event.Scroll.ScrollerSize.OnScroll_Touch",
+            scrolling_node->scroll_clip_layer_bounds.GetArea(), 1,
+            kMaxScrollerSize, kBucketNum);
+      }
+    }
   }
 
   if (scroll_on_main_thread) {