Revert "Temporarily disable performance.now() in the d8 shell."

src/d8.h

Index: src/d8.h
diff --git a/src/d8.h b/src/d8.h
index b8cb29379846567e0c08a9b3617b24c73ce4b501..db2edb93c98e5be4650447487c4e9130fd5776c2 100644
--- a/src/d8.h
+++ b/src/d8.h
@@ -300,6 +300,8 @@ class Shell : public i::AllStatic {
                                                 Handle<String> command);
   static void DispatchDebugMessages();
 #endif  // ENABLE_DEBUGGER_SUPPORT
+
+  static void PerformanceNow(const v8::FunctionCallbackInfo<v8::Value>& args);
 #endif  // V8_SHARED
 
   static void RealmCurrent(const v8::FunctionCallbackInfo<v8::Value>& args);
@@ -392,6 +394,7 @@ class Shell : public i::AllStatic {
   static CounterCollection* counters_;
   static i::OS::MemoryMappedFile* counters_file_;
   static i::Mutex context_mutex_;
+  static const i::TimeTicks kInitialTicks;
 
   static Counter* GetCounter(const char* name, bool is_histogram);
   static void InstallUtilityScript(Isolate* isolate);