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

Source/devtools/front_end/timeline/TracingTimelineModel.js

Index: Source/devtools/front_end/timeline/TracingTimelineModel.js
diff --git a/Source/devtools/front_end/timeline/TracingTimelineModel.js b/Source/devtools/front_end/timeline/TracingTimelineModel.js
index 5d06230c3f602282d1137c8e79dd0572f63725db..5a9e0357a981136899d803a3c8c06e2ce22b5b58 100644
--- a/Source/devtools/front_end/timeline/TracingTimelineModel.js
+++ b/Source/devtools/front_end/timeline/TracingTimelineModel.js
@@ -543,11 +543,12 @@ WebInspector.TracingTimelineModel.TraceEventRecord.prototype = {
     },
 
     /**
-     * @return {string|undefined}
+     * @return {string}
      */
     thread: function()
     {
-        return "CPU";
+        // FIXME: Should return the actual thread name.
+        return WebInspector.TimelineModel.MainThreadName;
     },
 
     /**