Issue repaints when offsetFromRenderer changes for any squashed layer.

Source/core/rendering/compositing/CompositedLayerMapping.h

Index: Source/core/rendering/compositing/CompositedLayerMapping.h
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.h b/Source/core/rendering/compositing/CompositedLayerMapping.h
index b2027caff6dbb27d7882ee18549ac7ede32df65d..262b1b6696a7fbd22be5e82edf04478af99f59f8 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.h
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.h
@@ -48,12 +48,16 @@ struct GraphicsLayerPaintInfo {
 
     // Offset describing where this squashed RenderLayer paints into the shared GraphicsLayer backing.
     IntSize offsetFromRenderer;
+    bool offsetFromRendererSet;
+
     LayoutSize subpixelAccumulation;
 
     GraphicsLayerPaintingPhase paintingPhase;
 
     bool isBackgroundLayer;
 
+    GraphicsLayerPaintInfo() : renderLayer(0), offsetFromRendererSet(false), isBackgroundLayer(false) { }
+
     bool isEquivalentForSquashing(const GraphicsLayerPaintInfo& other)
     {
         // FIXME: offsetFromRenderer and compositedBounds should not be checked here, because