Fix pixel snapping issue when transforming

Source/core/layout/Layer.cpp

Index: Source/core/layout/Layer.cpp
diff --git a/Source/core/layout/Layer.cpp b/Source/core/layout/Layer.cpp
index c34cc2b2340fcd9b828f5c8736b5e1b5cede54d8..7d89f35ff00d0b1fbddc5e7b60abc74fb45fd0b7 100644
--- a/Source/core/layout/Layer.cpp
+++ b/Source/core/layout/Layer.cpp
@@ -1628,6 +1628,7 @@ void Layer::collectFragments(LayerFragments& fragments, const Layer* rootLayer,
         ancestorClipRect.intersect(dirtyRect);
     }
 
+    const LayoutSize subPixelAccumulationIfNeeded = compositingState() == PaintsIntoOwnBacking ? LayoutSize() : subPixelAccumulation;
     for (size_t i = 0; i < fragments.size(); ++i) {
         LayerFragment& fragment = fragments.at(i);
 
@@ -1635,7 +1636,7 @@ void Layer::collectFragments(LayerFragments& fragments, const Layer* rootLayer,
         fragment.setRects(layerBoundsInFlowThread, backgroundRectInFlowThread, foregroundRectInFlowThread, outlineRectInFlowThread);
 
         // Shift to the root-relative physical position used when painting the flow thread in this fragment.
-        fragment.moveBy(fragment.paginationOffset + offsetOfPaginationLayerFromRoot);
+        fragment.moveBy(fragment.paginationOffset + offsetOfPaginationLayerFromRoot +  subPixelAccumulationIfNeeded);

[qiankun, 2015/03/10 01:37:00]

Hi Chris, this code is what I said fixed the multi-column tests:
virtual/regionbasedmulticol/fast/multicol/composited-with-overflow-in-next-column.html
virtual/slimmingpaint/fast/multicol/composited-with-overflow-in-next-column.html

 
         // Intersect the fragment with our ancestor's background clip so that e.g., columns in an overflow:hidden block are
         // properly clipped by the overflow.