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 9768a4e66bc8573ec6f2f0f31419c83faa4ed4ae..71d380fbc75cdcf76b60798dbd9d051a52158c36 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
@@ -130,35 +130,38 @@ 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 step = 0;
+float ScrollableArea::scrollStep(ScrollGranularity granularity, ScrollbarOrientation orientation) const
+{
     switch (granularity) {
     case ScrollByLine:
-        step = lineStep(orientation);
-        break;
+        return lineStep(orientation);
     case ScrollByPage:
-        step = pageStep(orientation);
-        break;
+        return pageStep(orientation);
     case ScrollByDocument:
-        step = documentStep(orientation);
-        break;
+        return documentStep(orientation);
     case ScrollByPixel:
     case ScrollByPrecisePixel:
-        step = pixelStep(orientation);
-        break;
+        return pixelStep(orientation);
+    default:
+        ASSERT_NOT_REACHED();
+        return 0.0f;
     }
+}
+
+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;