Fixed rounding issue on scrollbar rasterization.

cc/layers/painted_scrollbar_layer.h

Index: cc/layers/painted_scrollbar_layer.h
diff --git a/cc/layers/painted_scrollbar_layer.h b/cc/layers/painted_scrollbar_layer.h
index 4799bfb3fa14fd6504e6a06f02c6d306d619922d..c0e01ebbb7305c13f2e4ca0df763fde4f0a22bd1 100644
--- a/cc/layers/painted_scrollbar_layer.h
+++ b/cc/layers/painted_scrollbar_layer.h
@@ -77,7 +77,13 @@ class CC_EXPORT PaintedScrollbarLayer : public ScrollbarLayerInterface,
   int MaxTextureSize();
   float ClampScaleToMaxTextureSize(float scale);
 
-  UIResourceBitmap RasterizeScrollbarPart(const gfx::Rect& rect,
+  float calculateIdealScale(float content_scale,
+                            int layer_edge,
+                            int layer_size,
+                            int target_size) const;
+
+  UIResourceBitmap RasterizeScrollbarPart(const gfx::Rect& layer_rect,
+                                          const gfx::Rect& content_rect,
                                           ScrollbarPart part);
 
   scoped_ptr<Scrollbar> scrollbar_;