Fixed rounding issue on scrollbar rasterization.

cc/layers/painted_scrollbar_layer.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYERS_PAINTED_SCROLLBAR_LAYER_H_
 #define CC_LAYERS_PAINTED_SCROLLBAR_LAYER_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/input/scrollbar.h"
 #include "cc/layers/contents_scaling_layer.h"
@@ skipping 59 matching lines @@
   template<typename T> void UpdateProperty(T value, T* prop) {
     if (*prop == value)
       return;
     *prop = value;
     SetNeedsPushProperties();
   }
 
   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_;
   int scroll_layer_id_;
   int clip_layer_id_;
 
   // Snapshot of properties taken in UpdateThumbAndTrackGeometry and used in
   // PushPropertiesTo.
   int thumb_thickness_;
   int thumb_length_;
   gfx::Point location_;
   gfx::Rect track_rect_;
   bool is_overlay_;
   bool has_thumb_;
 
   scoped_ptr<ScopedUIResource> track_resource_;
   scoped_ptr<ScopedUIResource> thumb_resource_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintedScrollbarLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_PAINTED_SCROLLBAR_LAYER_H_