Timeline: do not imply event.thread.target is the main target

Source/devtools/front_end/timeline/TimelineModel.js

Index: Source/devtools/front_end/timeline/TimelineModel.js
diff --git a/Source/devtools/front_end/timeline/TimelineModel.js b/Source/devtools/front_end/timeline/TimelineModel.js
index eb1a1bc431de541eaadaaa79e947f0163d0eb72c..0a768dfee48b297b2e9f2f7b585eae4f5208c946 100644
--- a/Source/devtools/front_end/timeline/TimelineModel.js
+++ b/Source/devtools/front_end/timeline/TimelineModel.js
@@ -500,6 +500,15 @@ WebInspector.TimelineModel.prototype = {
     },
 
     /**
+     * @return {?WebInspector.Target}
+     */
+    target: function()
+    {
+        // FIXME: Consider returning null for loaded traces.
+        return this._tracingManager.target();
+    },
+
+    /**
      * @param {!Array.<!WebInspector.TracingManager.EventPayload>} events
      */
     setEventsForTest: function(events)