Add the SharedWorker.workerStart property for high resolution timers.

Source/modules/performance/WorkerPerformance.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 19 matching lines @@
 
 #include "config.h"
 #include "modules/performance/WorkerPerformance.h"
 
 #include "core/workers/DedicatedWorkerGlobalScope.h"
 #include "core/workers/WorkerGlobalScope.h"
 #include "wtf/CurrentTime.h"
 
 namespace WebCore {
 
-WorkerPerformance::WorkerPerformance(ExecutionContext* context)
-    : ContextLifecycleObserver(context)
+WorkerPerformance::WorkerPerformance()
 {
+    ScriptWrappable::init(this);
 }
 
 WorkerPerformance::~WorkerPerformance()
 {
 }
 
-double WorkerPerformance::now() const
+double WorkerPerformance::now(ExecutionContext* context) const
 {
-    ExecutionContext* context = executionContext();
     ASSERT(context);
+    ASSERT(context->isWorkerGlobalScope());
     WorkerGlobalScope* workerGlobalScope = toWorkerGlobalScope(context);
     return 1000.0 * (monotonicallyIncreasingTime() - workerGlobalScope->timeOrigin());
 }
 
 } // namespace WebCore