Unify memory optimization modes and flags.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 4dcafb2c6f39d02737fa085b647fb378808a6d1e..bc1fc0807fe26ae04d708be936bc0dccd976c323 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1162,6 +1162,12 @@ class Isolate {
 
   void SetRAILMode(RAILMode rail_mode);
 
+  void IsolateInForegroundNotification();
+
+  void IsolateInBackgroundNotification();
+
+  bool IsIsolateInBackground() { return is_isolate_in_background_; }
+
  protected:
   explicit Isolate(bool enable_serializer);
   bool IsArrayOrObjectPrototype(Object* object);
@@ -1353,6 +1359,10 @@ class Isolate {
   // True if ES2015 tail call elimination feature is enabled.
   bool is_tail_call_elimination_enabled_;
 
+  // True if the isolate is in background. This flag is used
+  // to prioritize between memory usage and latency.
+  bool is_isolate_in_background_;
+
   // Time stamp at initialization.
   double time_millis_at_init_;