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

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 05e54add9ff4d617bc0f5062f6bce38efc75ab54..27faef70bfb58beb9279927f1d6bc3014242db87 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -94,7 +94,8 @@ class CC_EXPORT LayerTreeImpl {
   gfx::Rect DeviceViewport() const;
   const gfx::Rect ViewportRectForTilePriority() const;
   std::unique_ptr<ScrollbarAnimationController>
-  CreateScrollbarAnimationController(ElementId scroll_element_id);
+  CreateScrollbarAnimationController(ElementId scroll_element_id,
+                                     float initial_opacity);
   void DidAnimateScrollOffset();
   bool use_gpu_rasterization() const;
   GpuRasterizationStatus GetGpuRasterizationStatus() const;
@@ -142,10 +143,6 @@ class CC_EXPORT LayerTreeImpl {
   LayerImplList::reverse_iterator rbegin();
   LayerImplList::reverse_iterator rend();
 
-  // TODO(crbug.com/702832): This won't be needed if overlay scrollbars have
-  // element ids.
-  void AddToOpacityAnimationsMap(int id, float opacity);
-
   void SetTransformMutated(ElementId element_id,
                            const gfx::Transform& transform);
   void SetOpacityMutated(ElementId element_id, float opacity);