[polymer] Switches .appendChild() to Polymer.dom()...appendChild()

tracing/tracing/ui/timeline_track_view.html

Index: tracing/tracing/ui/timeline_track_view.html
diff --git a/tracing/tracing/ui/timeline_track_view.html b/tracing/tracing/ui/timeline_track_view.html
index 90dd56bf58953f73a38445867d15caaaeadb33f8..8a7bd030f59a054b2d55db78e68b8b81b977d845 100644
--- a/tracing/tracing/ui/timeline_track_view.html
+++ b/tracing/tracing/ui/timeline_track_view.html
@@ -94,11 +94,11 @@ Polymer({
     this.rulerTrackContainer_.invalidate();
 
     this.rulerTrack_ = new tr.ui.tracks.RulerTrack(this.viewport_);
-    this.rulerTrackContainer_.appendChild(this.rulerTrack_);
+    Polymer.dom(this.rulerTrackContainer_).appendChild(this.rulerTrack_);
 
     this.upperModelTrack_ = new tr.ui.tracks.ModelTrack(this.viewport_);
     this.upperModelTrack_.upperMode = true;
-    this.rulerTrackContainer_.appendChild(this.upperModelTrack_);
+    Polymer.dom(this.rulerTrackContainer_).appendChild(this.upperModelTrack_);
 
     this.modelTrackContainer_ =
         new tr.ui.tracks.DrawingContainer(this.viewport_);
@@ -109,7 +109,7 @@ Polymer({
     this.viewport_.modelTrackContainer = this.modelTrackContainer_;
 
     this.modelTrack_ = new tr.ui.tracks.ModelTrack(this.viewport_);
-    this.modelTrackContainer_.appendChild(this.modelTrack_);
+    Polymer.dom(this.modelTrackContainer_).appendChild(this.modelTrack_);
 
     this.timingTool_ = new tr.ui.b.TimingTool(this.viewport_, this);