Revert of Remove indirection: setup scrollbar scroll layers in the scrollbar constructor

cc/layers/scrollbar_layer_unittest.cc

Index: cc/layers/scrollbar_layer_unittest.cc
diff --git a/cc/layers/scrollbar_layer_unittest.cc b/cc/layers/scrollbar_layer_unittest.cc
index 0135d556bdff4237a05f03beae43ba710be42c26..8824b2880d9ecb1b055a02837cfc7a247432d1a2 100644
--- a/cc/layers/scrollbar_layer_unittest.cc
+++ b/cc/layers/scrollbar_layer_unittest.cc
@@ -308,6 +308,7 @@
 
   root_layer->SetScrollOffset(gfx::ScrollOffset(0, 0));
   scrollbar_layer->SetBounds(gfx::Size(70, 10));
+  scrollbar_layer->SetScrollLayer(root_layer->id());
   scrollbar_layer->fake_scrollbar()->set_location(gfx::Point(20, 10));
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(30, 10, 50, 10));
   scrollbar_layer->fake_scrollbar()->set_thumb_thickness(10);
@@ -346,6 +347,7 @@
 
   root_layer->SetScrollOffset(gfx::ScrollOffset(0, 0));
   scrollbar_layer->SetBounds(gfx::Size(70, 10));
+  scrollbar_layer->SetScrollLayer(root_layer->id());
   scrollbar_layer->fake_scrollbar()->set_location(gfx::Point(20, 10));
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(30, 10, 50, 10));
   scrollbar_layer->fake_scrollbar()->set_thumb_thickness(10);
@@ -419,6 +421,7 @@
 
   root_layer->SetScrollOffset(gfx::ScrollOffset(0, 0));
   scrollbar_layer->SetBounds(gfx::Size(10, 20));
+  scrollbar_layer->SetScrollLayer(root_layer->id());
   scrollbar_layer->fake_scrollbar()->set_track_rect(gfx::Rect(0, 0, 10, 20));
   scrollbar_layer->fake_scrollbar()->set_thumb_thickness(10);
   scrollbar_layer->fake_scrollbar()->set_thumb_length(4);