Add support for setting TrackStart on solid color layers.

cc/layers/solid_color_scrollbar_layer_impl_unittest.cc

Index: cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
diff --git a/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc b/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
index 0317015e96e4806887a8cb13e178ff4c8da217dc..ba82be53574df103f55efd131d59404afc73f082 100644
--- a/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
+++ b/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
@@ -18,6 +18,7 @@ TEST(SolidColorScrollbarLayerImplTest, Occlusion) {
 
   ScrollbarOrientation orientation = VERTICAL;
   int thumb_thickness = layer_size.width();
+  int track_start = 0;
   bool is_left_side_vertical_scrollbar = false;
   bool is_overlay = false;
 
@@ -25,6 +26,7 @@ TEST(SolidColorScrollbarLayerImplTest, Occlusion) {
       impl.AddChildToRoot<SolidColorScrollbarLayerImpl>(
           orientation,
           thumb_thickness,
+          track_start,
           is_left_side_vertical_scrollbar,
           is_overlay);
   scrollbar_layer_impl->SetAnchorPoint(gfx::PointF());