Introduce new API for detecting when the embedder is in foreground/background.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index ba8de1cc9c70aee427f8566dfd884c3eae736619..57154a0b33b3b832d7248691fe7fdaf5a5b67bef 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7367,6 +7367,18 @@ int Isolate::ContextDisposedNotification(bool dependant_context) {
 }
 
 
+void Isolate::IsolateInForegroundNotification() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  return isolate->heap()->SetOptimizeForLatency();
+}
+
+
+void Isolate::IsolateInBackgroundNotification() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  return isolate->heap()->SetOptimizeForMemoryUsage();
+}
+
+
 void Isolate::SetJitCodeEventHandler(JitCodeEventOptions options,
                                      JitCodeEventHandler event_handler) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);