DevTools: Unify main thread name across vanilla and tracing timeline modes.

Source/devtools/front_end/timeline/TimelineModelImpl.js

Index: Source/devtools/front_end/timeline/TimelineModelImpl.js
diff --git a/Source/devtools/front_end/timeline/TimelineModelImpl.js b/Source/devtools/front_end/timeline/TimelineModelImpl.js
index 54096f95e9ebb25c004352178236a7973be9bd7f..03828072a23d00639fc0cf019bd5e2763b27559b 100644
--- a/Source/devtools/front_end/timeline/TimelineModelImpl.js
+++ b/Source/devtools/front_end/timeline/TimelineModelImpl.js
@@ -299,6 +299,7 @@ WebInspector.TimelineModel.RecordImpl = function(model, timelineEvent, parentRec
     this._model = model;
     var bindings = this._model._bindings;
     this._record = timelineEvent;
+    this._thread = this._record.thread || WebInspector.TimelineModel.MainThreadName;
     this._children = [];
     if (parentRecord) {
         this.parent = parentRecord;
@@ -422,11 +423,11 @@ WebInspector.TimelineModel.RecordImpl.prototype = {
     },
 
     /**
-     * @return {string|undefined}
+     * @return {string}
      */
     thread: function()
     {
-        return this._record.thread;
+        return this._thread;
     },
 
     /**