Scrollbar ThumbLength is not updated when window size is changed.

cc/test/fake_scrollbar.h

 // Copyright 2012 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_TEST_FAKE_SCROLLBAR_H_
 #define CC_TEST_FAKE_SCROLLBAR_H_
 
 #include "base/compiler_specific.h"
 #include "cc/input/scrollbar.h"
 #include "third_party/skia/include/core/SkColor.h"
@@ skipping 18 matching lines @@
   virtual void PaintPart(SkCanvas* canvas,
                          ScrollbarPart part,
                          const gfx::Rect& content_rect) OVERRIDE;
 
   void set_location(const gfx::Point& location) { location_ = location; }
   void set_track_rect(const gfx::Rect& track_rect) { track_rect_ = track_rect; }
   void set_thumb_thickness(int thumb_thickness) {
       thumb_thickness_ = thumb_thickness;
   }
   void set_thumb_length(int thumb_length) { thumb_length_ = thumb_length; }
+  void set_has_thumb(bool has_thumb) { has_thumb_ = has_thumb; }
+
   SkColor paint_fill_color() const { return SK_ColorBLACK | fill_color_; }
 
  private:
   bool paint_;
   bool has_thumb_;
   bool is_overlay_;
   int thumb_thickness_;
   int thumb_length_;
   gfx::Point location_;
   gfx::Rect track_rect_;
   SkColor fill_color_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeScrollbar);
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_FAKE_SCROLLBAR_H_