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

cc/blink/web_scrollbar_layer_impl.cc

Index: cc/blink/web_scrollbar_layer_impl.cc
diff --git a/cc/blink/web_scrollbar_layer_impl.cc b/cc/blink/web_scrollbar_layer_impl.cc
index e966ab05047805d20d8ff4f326114f341f244ec3..0db9d0f5c5de1f8d42487f0ea3b8e173e9f6f143 100644
--- a/cc/blink/web_scrollbar_layer_impl.cc
+++ b/cc/blink/web_scrollbar_layer_impl.cc
@@ -75,4 +75,8 @@ void WebScrollbarLayerImpl::SetScrollLayer(blink::WebLayer* layer) {
       scroll_layer ? scroll_layer->element_id() : cc::ElementId());
 }
 
+void WebScrollbarLayerImpl::SetElementId(const cc::ElementId& element_id) {
+  layer_->SetElementId(element_id);
+}
+
 }  // namespace cc_blink