Scrollbar ThumbLength is not updated when window size is changed.

cc/test/fake_scrollbar.h

Index: cc/test/fake_scrollbar.h
diff --git a/cc/test/fake_scrollbar.h b/cc/test/fake_scrollbar.h
index 872b9d95ab7312f5255164e212a42ad24fff6516..4084f9ca93c1ca3a54f536fd3e1fe6895a81b147 100644
--- a/cc/test/fake_scrollbar.h
+++ b/cc/test/fake_scrollbar.h
@@ -36,6 +36,8 @@ class FakeScrollbar : public Scrollbar {
       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: