Add the rate of js invocations from the api as a signal of idleness

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 8f4915cefd55bbab5f0f19b70a41ad7a08a8e734..522d617e6bca3693b2b8a0b6c818b8c15f2994a5 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1046,6 +1046,12 @@ class Isolate {
     return id;
   }
 
+  void IncrementJsCallsFromApiCounter() { ++js_calls_from_api_counter_; }
+
+  unsigned int js_calls_from_api_counter() {
+    return js_calls_from_api_counter_;
+  }
+
   // Get (and lazily initialize) the registry for per-isolate symbols.
   Handle<JSObject> GetSymbolRegistry();
 
@@ -1313,6 +1319,9 @@ class Isolate {
 
   int next_optimization_id_;
 
+  // Counts javascript calls from the API. Wraps around on overflow.
+  unsigned int js_calls_from_api_counter_;
+
 #if TRACE_MAPS
   int next_unique_sfi_id_;
 #endif