Make number of available threads isolate-dependent and expose it to ResourceConstraints.

src/optimizing-compiler-thread.h

Index: src/optimizing-compiler-thread.h
diff --git a/src/optimizing-compiler-thread.h b/src/optimizing-compiler-thread.h
index 754aecebf5c9b17b88a1f2017254ece53411206e..795fa65588ab894ef7d7fcb0c95e96a748c4331c 100644
--- a/src/optimizing-compiler-thread.h
+++ b/src/optimizing-compiler-thread.h
@@ -96,7 +96,12 @@ class OptimizingCompilerThread : public Thread {
     AddToOsrBuffer(NULL);
   }
 
+  static bool Enabled(int max_available) {
+    return (FLAG_concurrent_recompilation && max_available > 1);
+  }
+
 #ifdef DEBUG
+  static bool IsOptimizerThread(Isolate* isolate);
   bool IsOptimizerThread();
 #endif