Record size of scroller that user scrolls

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2733 matching lines @@
       }
     }
 
     auto* scrolling_layer = FindScrollLayerForDeviceViewportPoint(
         device_viewport_point, type, layer_impl, &scroll_on_main_thread,
         &scroll_status.main_thread_scrolling_reasons);
     ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
     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) {
     RecordCompositorSlowScrollMetric(type, MAIN_THREAD);
 
     scroll_status.thread = SCROLL_ON_MAIN_THREAD;
     return scroll_status;
   } else if (scrolling_node) {
     scroll_affects_scroll_handler_ = active_tree_->have_scroll_event_handlers();
   }
@@ skipping 1511 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc