Make compositor and blink talk in fractional scroll offset

Source/platform/graphics/GraphicsLayer.cpp

Index: Source/platform/graphics/GraphicsLayer.cpp
diff --git a/Source/platform/graphics/GraphicsLayer.cpp b/Source/platform/graphics/GraphicsLayer.cpp
index 0f7d6acd45c69d18c37e9cd14414cc07982741da..2d3a0209d7f2ac220e63b5c54b23f7c40bc6c12d 100644
--- a/Source/platform/graphics/GraphicsLayer.cpp
+++ b/Source/platform/graphics/GraphicsLayer.cpp
@@ -1057,8 +1057,10 @@ void GraphicsLayer::notifyAnimationFinished(double, WebCompositorAnimation::Targ
 
 void GraphicsLayer::didScroll()
 {
-    if (m_scrollableArea)
-        m_scrollableArea->scrollToOffsetWithoutAnimation(m_scrollableArea->minimumScrollPosition() + toIntSize(m_layer->layer()->scrollPosition()));
+    if (m_scrollableArea) {
+        DoublePoint newPosition = m_scrollableArea->minimumScrollPosition() + toDoubleSize(m_layer->layer()->scrollPositionDouble());
+        m_scrollableArea->scrollToOffsetWithoutAnimation(toFloatPoint(newPosition));

[Rick Byers, 2014/10/04 00:04:24]

FIXME to remove the 'toFloatPoint' (like all the other callsites).  Hopefully
that's an easy cleanup to do soon (so we can stop adding new callsites).

[Yufeng Shen (Slow to review), 2014/10/06 17:54:58]

Done.

+    }
 }
 
 } // namespace blink