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 344eb45d30a9fc8c238c528fb221c88687111196..347f337967b06cbfed370a782c6e10ac0be2cddb 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"
@@ -2795,6 +2796,20 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     scrolling_node =
         scrolling_layer ? scroll_tree.Node(scrolling_layer->scroll_tree_index())
                         : nullptr;
+    if (!scroll_on_main_thread && scrolling_node &&
+        !viewport()->MainScrollLayer()) {

[bokan, 2017/04/18 19:24:38]

It'd be safer to check that the node is neither the inner, nor the outer,
viewport layer -- rather than viewport()->MainScrollLayer()

Also, right now, this is just checking that no viewport layer exists (where I
think you mean to compare to scrolling_node) so you'll never record here. Please
add a test that checks the CC side of the metric too.

[yigu, 2017/04/19 20:53:05]

Done.

+      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) {