Add metric to know how user use scrollbar

third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.cpp
index 36899a94016fae97c00e6de3150deec10e136cdd..0fdbed2aaae7b32e9b01dd77ffe4bdaedc10b9eb 100644
--- a/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintInvalidationCapableScrollableArea.cpp
@@ -5,6 +5,7 @@
 #include "core/paint/PaintInvalidationCapableScrollableArea.h"
 
 #include "core/frame/Settings.h"
+#include "core/frame/UseCounter.h"
 #include "core/html/HTMLFrameOwnerElement.h"
 #include "core/layout/LayoutBox.h"
 #include "core/layout/LayoutScrollbar.h"
@@ -204,4 +205,38 @@ void PaintInvalidationCapableScrollableArea::
   layoutBox()->setMayNeedPaintInvalidation();
 }
 
+void PaintInvalidationCapableScrollableArea::didScrollWithScrollbar(
+    ScrollbarPart part,
+    ScrollbarOrientation orientation) {
+  UseCounter::Feature scrollbarUseUMA;
+  switch (part) {
+    case BackButtonStartPart:
+    case ForwardButtonStartPart:
+    case BackButtonEndPart:
+    case ForwardButtonEndPart:
+      scrollbarUseUMA =
+          (orientation == VerticalScrollbar
+               ? UseCounter::ScrollbarUseVerticalScrollbarButton
+               : UseCounter::ScrollbarUseHorizontalScrollbarButton);
+      break;
+    case ThumbPart:
+      scrollbarUseUMA =
+          (orientation == VerticalScrollbar
+               ? UseCounter::ScrollbarUseVerticalScrollbarThumb
+               : UseCounter::ScrollbarUseHorizontalScrollbarThumb);
+      break;
+    case BackTrackPart:
+    case ForwardTrackPart:
+      scrollbarUseUMA =
+          (orientation == VerticalScrollbar
+               ? UseCounter::ScrollbarUseVerticalScrollbarTrack
+               : UseCounter::ScrollbarUseHorizontalScrollbarTrack);
+      break;
+    default:
+      return;
+  }
+
+  UseCounter::count(layoutBox()->document(), scrollbarUseUMA);
+}
+
 }  // namespace blink