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 3c3850326f2d3a2a4465f3b296d1cd1ad142ecc4..add80a24cd575be6173fb56ff1259557d9d40a0a 100644
--- a/Source/devtools/front_end/timeline/TimelineModel.js
+++ b/Source/devtools/front_end/timeline/TimelineModel.js
@@ -499,6 +499,15 @@ 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)