Handle gesture scroll events on scrollbars from Touchpad.

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

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index f1fa7bc92c109843ae33199d7aa3b73c4bef4c0d..b340a59ad9af33bc6f118e92e25edce2588d0610 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -325,25 +325,60 @@ void Scrollbar::setPressedPart(ScrollbarPart part)
     m_pressedPart = part;
 }
 
-bool Scrollbar::gestureEvent(const PlatformGestureEvent& evt)
+bool Scrollbar::gestureEvent(const PlatformGestureEvent& evt, bool* shouldUpdateCapture)
 {
+    DCHECK(shouldUpdateCapture);
     switch (evt.type()) {
     case PlatformEvent::GestureTapDown:
         setPressedPart(theme().hitTest(*this, evt.position()));
         m_pressedPos = orientation() == HorizontalScrollbar ? convertFromRootFrame(evt.position()).x() : convertFromRootFrame(evt.position()).y();
+        *shouldUpdateCapture = true;
         return true;
     case PlatformEvent::GestureTapDownCancel:
-    case PlatformEvent::GestureScrollBegin:
         if (m_pressedPart != ThumbPart)
             return false;
         m_scrollPos = m_pressedPos;
         return true;
+    case PlatformEvent::GestureScrollBegin:
+        switch (evt.source()) {
+        case PlatformGestureSourceTouchpad:
+            // Update the state on GSB for touchpad since GestureTapDown
+            // is not generated by that device. Touchscreen uses the tap down
+            // gesture since the scrollbar enters a visual active state.
+            *shouldUpdateCapture = true;
+            setPressedPart(NoPart);
+            m_pressedPos = 0;
+            return true;
+        case PlatformGestureSourceTouchscreen:
+            if (m_pressedPart != ThumbPart)
+                return false;
+            m_scrollPos = m_pressedPos;
+            return true;
+        default:
+            ASSERT_NOT_REACHED();
+            return true;
+        }
+        break;
     case PlatformEvent::GestureScrollUpdate:
-        if (m_pressedPart != ThumbPart)
+        switch (evt.source()) {
+        case PlatformGestureSourceTouchpad: {
+            FloatSize delta(-evt.deltaX(), -evt.deltaY());
+            if (m_scrollableArea && m_scrollableArea->userScroll(evt.deltaUnits(), delta).didScroll()) {
+                return true;
+            }
             return false;
-        m_scrollPos += orientation() == HorizontalScrollbar ? evt.deltaX() : evt.deltaY();
-        moveThumb(m_scrollPos, false);
-        return true;
+        }
+        case PlatformGestureSourceTouchscreen:
+            if (m_pressedPart != ThumbPart)
+                return false;
+            m_scrollPos += orientation() == HorizontalScrollbar ? evt.deltaX() : evt.deltaY();
+            moveThumb(m_scrollPos, false);
+            return true;
+        default:
+            ASSERT_NOT_REACHED();
+            return true;
+        }
+        break;
     case PlatformEvent::GestureScrollEnd:
     case PlatformEvent::GestureLongPress:
     case PlatformEvent::GestureFlingStart: