Add support for setting TrackStart on solid color layers.

cc/layers/solid_color_scrollbar_layer.h

Index: cc/layers/solid_color_scrollbar_layer.h
diff --git a/cc/layers/solid_color_scrollbar_layer.h b/cc/layers/solid_color_scrollbar_layer.h
index 67d4ae7b36d92f9d4900330c02384fea37e9e8e5..654a6a6ae744add082134e5bffd089160e894b0f 100644
--- a/cc/layers/solid_color_scrollbar_layer.h
+++ b/cc/layers/solid_color_scrollbar_layer.h
@@ -20,6 +20,7 @@ class CC_EXPORT SolidColorScrollbarLayer : public ScrollbarLayerInterface,
   static scoped_refptr<SolidColorScrollbarLayer> Create(
       ScrollbarOrientation orientation,
       int thumb_thickness,
+      int track_start,
       bool is_left_side_vertical_scrollbar,
       int scroll_layer_id);
 
@@ -42,6 +43,7 @@ class CC_EXPORT SolidColorScrollbarLayer : public ScrollbarLayerInterface,
  protected:
   SolidColorScrollbarLayer(ScrollbarOrientation orientation,
                            int thumb_thickness,
+                           int track_start,
                            bool is_left_side_vertical_scrollbar,
                            int scroll_layer_id);
   virtual ~SolidColorScrollbarLayer();
@@ -51,6 +53,7 @@ class CC_EXPORT SolidColorScrollbarLayer : public ScrollbarLayerInterface,
   int clip_layer_id_;
   ScrollbarOrientation orientation_;
   int thumb_thickness_;
+  int track_start_;
   bool is_left_side_vertical_scrollbar_;
 
   DISALLOW_COPY_AND_ASSIGN(SolidColorScrollbarLayer);