Add performance.now() to the d8 shell.

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 96407a897c98b1284a3f9b1dd78bb457c5b08bd7..0efaeb76af56f18e73ecceb23a898b9351aa2706 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -166,6 +166,7 @@ Persistent<Context> Shell::utility_context_;
 Persistent<Context> Shell::evaluation_context_;
 ShellOptions Shell::options;
 const char* Shell::kPrompt = "d8> ";
+const i::TimeTicks Shell::kInitialTicks = i::TimeTicks::HighResolutionNow();
 
 
 const int MB = 1024 * 1024;
@@ -289,6 +290,15 @@ int PerIsolateData::RealmFind(Handle<Context> context) {
 }
 
 
+// window.performance.now() returns a time stamp as double, measured in
+// milliseconds.
+void Shell::WindowPerformanceNow(
+    const v8::FunctionCallbackInfo<v8::Value>& args) {
+  i::TimeDelta delta = i::TimeTicks::HighResolutionNow() - kInitialTicks;
+  args.GetReturnValue().Set(delta.InMillisecondsF());
+}
+
+
 // Realm.current() returns the index of the currently active realm.
 void Shell::RealmCurrent(const v8::FunctionCallbackInfo<v8::Value>& args) {
   Isolate* isolate = args.GetIsolate();
@@ -872,6 +882,13 @@ Handle<ObjectTemplate> Shell::CreateGlobalTemplate(Isolate* isolate) {
                               RealmSharedGet, RealmSharedSet);
   global_template->Set(String::New("Realm"), realm_template);
 
+  Handle<ObjectTemplate> window_template = ObjectTemplate::New();
+  Handle<ObjectTemplate> performance_template = ObjectTemplate::New();
+  performance_template->Set(String::New("now"),
+                            FunctionTemplate::New(WindowPerformanceNow));
+  window_template->Set(String::New("performance"), performance_template);
+  global_template->Set(String::New("window"), window_template);
+
 #if !defined(V8_SHARED) && !defined(_WIN32) && !defined(_WIN64)
   Handle<ObjectTemplate> os_templ = ObjectTemplate::New();
   AddOSMethods(os_templ);