Update RootFrameViewport::userScroll to distribute scrolls between viewports.

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

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
index 14086dc9317f532ec5a0990aaaec68201984456a..0c606f8677f83dc20e4d44b6a7b86a7a64340c80 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
@@ -130,19 +130,13 @@ GraphicsLayer* ScrollableArea::layerForContainer() const
     return layerForScrolling() ? layerForScrolling()->parent() : 0;
 }
 
-ScrollResultOneDimensional ScrollableArea::userScroll(ScrollDirectionPhysical direction, ScrollGranularity granularity, float delta)
+ScrollbarOrientation ScrollableArea::scrollbarOrientationFromDirection(ScrollDirectionPhysical direction) const
 {
-    ScrollbarOrientation orientation;
-    if (direction == ScrollUp || direction == ScrollDown)
-        orientation = VerticalScrollbar;
-    else
-        orientation = HorizontalScrollbar;
-
-    if (!userInputScrollable(orientation))
-        return ScrollResultOneDimensional(false, delta);
-
-    cancelProgrammaticScrollAnimation();
+    return (direction == ScrollUp  || direction == ScrollDown) ? VerticalScrollbar : HorizontalScrollbar;
+}
 
+float ScrollableArea::scrollStep(ScrollGranularity granularity, ScrollbarOrientation orientation) const
+{
     float step = 0;
     switch (granularity) {
     case ScrollByLine:
@@ -159,6 +153,18 @@ ScrollResultOneDimensional ScrollableArea::userScroll(ScrollDirectionPhysical di
         step = pixelStep(orientation);
         break;
     }
+    return step;
+}
+
+ScrollResultOneDimensional ScrollableArea::userScroll(ScrollDirectionPhysical direction, ScrollGranularity granularity, float delta)
+{
+    ScrollbarOrientation orientation = scrollbarOrientationFromDirection(direction);
+    if (!userInputScrollable(orientation))
+        return ScrollResultOneDimensional(false, delta);
+
+    cancelProgrammaticScrollAnimation();
+
+    float step = scrollStep(granularity, orientation);
 
     if (direction == ScrollUp || direction == ScrollLeft)
         delta = -delta;