Fix pixel snapping issue when transforming

Source/core/layout/Layer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1610 matching lines @@
         const Layer* ancestorLayer = rootLayerIsInsidePaginationLayer ? paginationParentLayer : rootLayer;
         ClipRectsContext clipRectsContext(ancestorLayer, clipRectsCacheSlot, inOverlayScrollbarSizeRelevancy);
         if (respectOverflowClip == IgnoreOverflowClip)
             clipRectsContext.setIgnoreOverflowClip();
         ancestorClipRect = enclosingPaginationLayer()->clipper().backgroundClipRect(clipRectsContext);
         if (rootLayerIsInsidePaginationLayer)
             ancestorClipRect.moveBy(-rootLayer->visualOffsetFromAncestor(ancestorLayer));
         ancestorClipRect.intersect(dirtyRect);
     }
 
+    const LayoutSize subPixelAccumulationIfNeeded = compositingState() == PaintsIntoOwnBacking ? LayoutSize() : subPixelAccumulation;
     for (size_t i = 0; i < fragments.size(); ++i) {
         LayerFragment& fragment = fragments.at(i);
 
         // Set our four rects with all clipping applied that was internal to the flow thread.
         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.
         fragment.intersect(ancestorClipRect.rect());
 
         // Now intersect with our pagination clip. This will typically mean we're just intersecting the dirty rect with the column
         // clip, so the column clip ends up being all we apply.
         fragment.intersect(fragment.paginationClip);
     }
 }
@@ skipping 1304 matching lines @@
     }
 }
 
 void showLayerTree(const blink::LayoutObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif