[tracing] Defer setting model until timeline-view is attached

tracing/tracing/ui/timeline_view.html

Index: tracing/tracing/ui/timeline_view.html
diff --git a/tracing/tracing/ui/timeline_view.html b/tracing/tracing/ui/timeline_view.html
index 88a9419e73161ab00f4ae7e7dc76b5bbfcb50388..6b28c10e89ba48f33ec5497994025610ed7ca423 100644
--- a/tracing/tracing/ui/timeline_view.html
+++ b/tracing/tracing/ui/timeline_view.html
@@ -144,8 +144,12 @@ Polymer({
     this.async(function() {
       this.trackViewContainer_ = Polymer.dom(this).querySelector(
           '#track_view_container');
-      if (!this.trackViewContainer_)
+      if (!this.trackViewContainer_) {
         console.error('missing trackviewContainer');
+      }
+      if (this.model_) {
+        this.updateContents_();
+      }
     });
   },
 
@@ -364,14 +368,18 @@ Polymer({
   },
 
   get model() {
-    if (this.trackView_)
-      return this.trackView_.model;
-    return undefined;
+    return this.model_;
   },
 
   set model(model) {
-    var modelInstanceChanged = model !== this.model;
-    var modelValid = model && !model.bounds.isEmpty;
+    this.model_ = model;
+    if (this.isAttached) this.updateContents_();
+  },
+
+  updateContents_: function() {
+    var modelInstanceChanged = (this.trackViewModel_ === undefined) ||
+        (this.model !== this.trackViewModel_.model);
+    var modelValid = this.model && !this.model.bounds.isEmpty;
 
     var importWarningsEl = Polymer.dom(this.root).querySelector(
         '#import-warnings');
@@ -406,17 +414,18 @@ Polymer({
 
     // Set the model.
     if (modelValid) {
-      this.trackView_.model = model;
+      this.trackView_.model = this.model;
       this.trackView_.viewport.showFlowEvents = this.showFlowEvents;
       this.trackView_.viewport.highlightVSync = this.highlightVSync;
-      if (this.railScoreSpan_)
-        this.railScoreSpan_.model = model;
-
-      this.$.display_unit.preferredTimeDisplayMode = model.intrinsicTimeUnit;
+      if (this.railScoreSpan_) {
+        this.railScoreSpan_.model = this.model;
+      }
+      this.$.display_unit.preferredTimeDisplayMode =
+        this.model.intrinsicTimeUnit;
     }
 
-    if (model) {
-      model.importWarningsThatShouldBeShownToUser.forEach(
+    if (this.model) {
+      this.model.importWarningsThatShouldBeShownToUser.forEach(
           function(importWarning) {
             importWarningsEl.addMessage(
                 'Import Warning: ' + importWarning.type + ': ' +