Fix cc scrollbar layer issues with initialization, and use element ids throughout.

cc/layers/painted_overlay_scrollbar_layer.cc

Index: cc/layers/painted_overlay_scrollbar_layer.cc
diff --git a/cc/layers/painted_overlay_scrollbar_layer.cc b/cc/layers/painted_overlay_scrollbar_layer.cc
index 5ab2eaed4b938f14d273942e007ea15fb70e6885..52ead4b4cb89ea003cc5ec796be358d743e9efd7 100644
--- a/cc/layers/painted_overlay_scrollbar_layer.cc
+++ b/cc/layers/painted_overlay_scrollbar_layer.cc
@@ -65,10 +65,6 @@ bool PaintedOverlayScrollbarLayer::OpacityCanAnimateOnImplThread() const {
   return scrollbar_->IsOverlay();
 }
 
-bool PaintedOverlayScrollbarLayer::AlwaysUseActiveTreeOpacity() const {
-  return true;
-}
-
 ScrollbarOrientation PaintedOverlayScrollbarLayer::orientation() const {
   return scrollbar_->Orientation();
 }