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

cc/layers/painted_scrollbar_layer.cc

Index: cc/layers/painted_scrollbar_layer.cc
diff --git a/cc/layers/painted_scrollbar_layer.cc b/cc/layers/painted_scrollbar_layer.cc
index 0f2e083d0f9974cbc0cc7289c8f521fb9ccc39a0..66dde4c29897c977d28be7703c92fda64d7176b5 100644
--- a/cc/layers/painted_scrollbar_layer.cc
+++ b/cc/layers/painted_scrollbar_layer.cc
@@ -73,10 +73,6 @@ bool PaintedScrollbarLayer::OpacityCanAnimateOnImplThread() const {
   return scrollbar_->IsOverlay();
 }
 
-bool PaintedScrollbarLayer::AlwaysUseActiveTreeOpacity() const {
-  return true;
-}
-
 ScrollbarOrientation PaintedScrollbarLayer::orientation() const {
   return scrollbar_->Orientation();
 }