Offer User Timing in workers

Source/modules/performance/WorkerPerformance.cpp

Index: Source/modules/performance/WorkerPerformance.cpp
diff --git a/Source/modules/performance/WorkerPerformance.cpp b/Source/modules/performance/WorkerPerformance.cpp
index d3e0c3da7b514a88abcc4ea77636ac0861911f99..5fd9b15228f91884fb59f9d5280dcb0a4c9fb438 100644
--- a/Source/modules/performance/WorkerPerformance.cpp
+++ b/Source/modules/performance/WorkerPerformance.cpp
@@ -38,21 +38,22 @@
 
 namespace blink {
 
-WorkerPerformance::WorkerPerformance()
+WorkerPerformance::WorkerPerformance(WorkerGlobalScope* context)
+    : PerformanceBase(context->timeOrigin())
+    , ContextLifecycleObserver(context)
 {
 }
 
-DEFINE_TRACE(WorkerPerformance)
+ExecutionContext* WorkerPerformance::executionContext() const
 {
-    visitor->trace(m_memoryInfo);
+    return ContextLifecycleObserver::executionContext();
 }
 
-double WorkerPerformance::now(ExecutionContext* context) const
+DEFINE_TRACE(WorkerPerformance)
 {
-    ASSERT(context);
-    ASSERT(context->isWorkerGlobalScope());
-    WorkerGlobalScope* workerGlobalScope = toWorkerGlobalScope(context);
-    return 1000.0 * (monotonicallyIncreasingTime() - workerGlobalScope->timeOrigin());
+    visitor->trace(m_memoryInfo);
+    PerformanceBase::trace(visitor);
+    ContextLifecycleObserver::trace(visitor);
 }
 
 MemoryInfo* WorkerPerformance::memory()