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

cc/blink/web_scrollbar_layer_impl.h

Index: cc/blink/web_scrollbar_layer_impl.h
diff --git a/cc/blink/web_scrollbar_layer_impl.h b/cc/blink/web_scrollbar_layer_impl.h
index 952277635c02f42e40e9cdc134355ba4914ccf1b..e8e0fe438aa5ea4549c4afa98c43e527491a7441 100644
--- a/cc/blink/web_scrollbar_layer_impl.h
+++ b/cc/blink/web_scrollbar_layer_impl.h
@@ -39,6 +39,8 @@ class WebScrollbarLayerImpl : public blink::WebScrollbarLayer {
   blink::WebLayer* Layer() override;
   void SetScrollLayer(blink::WebLayer* layer) override;
 
+  void SetElementId(const cc::ElementId&) override;

[wkorman, 2017/05/12 22:29:38]

Is it worth using ref param given struct is basically an int? Am fine either
way, I think others have commented similarly to me in past so figured to ask and
learn again from this reviewer set.

+
  private:
   std::unique_ptr<WebLayerImpl> layer_;