Add metric to know how user use scrollbar

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

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 #include "platform/scroll/Scrollbar.h"
 
-#include <algorithm>
+#include "platform/Histogram.h"
 #include "platform/HostWindow.h"
 #include "platform/PlatformMouseEvent.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/graphics/paint/CullRect.h"
 #include "platform/scroll/ScrollAnimatorBase.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "platform/scroll/ScrollbarTheme.h"
 #include "public/platform/WebGestureEvent.h"
 
 namespace blink {
@@ skipping 271 matching lines @@
   m_hoveredPart = part;
 }
 
 void Scrollbar::setPressedPart(ScrollbarPart part) {
   if (m_pressedPart != NoPart
       // When we no longer have a pressed part, we can start drawing a hovered
       // state on the hovered part.
       || m_hoveredPart != NoPart)
     setNeedsPaintInvalidation(
         static_cast<ScrollbarPart>(m_pressedPart | m_hoveredPart | part));
+
+  // Send ScrollbarPressedPart histogram.
+  ScrollbarPressedPart scrollbarPressedPart = IgnorePart;

[bokan, 2017/01/16 21:25:33]

Put all of this into a helper method that you call from here.

+  switch (part) {
+    case BackButtonStartPart:
+    case ForwardButtonStartPart:
+    case BackButtonEndPart:
+    case ForwardButtonEndPart:
+      scrollbarPressedPart =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarButton
+                                              : HorizontalScrollbarButton);
+      break;
+    case ThumbPart:
+      scrollbarPressedPart =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarThumb
+                                              : HorizontalScrollbarThumb);
+      break;
+    case BackTrackPart:
+    case ForwardTrackPart:
+      scrollbarPressedPart =
+          (orientation() == VerticalScrollbar ? VerticalScrollbarTrack
+                                              : HorizontalScrollbarTrack);
+      break;
+    default:
+      scrollbarPressedPart = IgnorePart;
+  }
+
+  if (scrollbarPressedPart != IgnorePart) {
+    DEFINE_STATIC_LOCAL(EnumerationHistogram, scrollbarPressedPartHistogram,
+                        ("ScrollbarPressedPart", ScrollbarPressedPartMax));
+    scrollbarPressedPartHistogram.count(

[bokan, 2017/01/16 21:25:33]

This will still count multiple uses on a page load. See
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/frame/Vis...
how I made sure that we only report the viewport stats once per page load by
recording them at destruction time. (I'll confirm that we fixed the bug
regarding UMA stats collected at destruction)

+        static_cast<ScrollbarPressedPart>(scrollbarPressedPart));
+  }
+
   m_pressedPart = part;
 }
 
 bool Scrollbar::gestureEvent(const WebGestureEvent& evt,
                              bool* shouldUpdateCapture) {
   DCHECK(shouldUpdateCapture);
   switch (evt.type()) {
     case WebInputEvent::GestureTapDown: {
       IntPoint position = flooredIntPoint(evt.positionInRootFrame());
       setPressedPart(theme().hitTest(*this, position));
@@ skipping 299 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink