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

src/optimizing-compiler-thread.cc

Index: src/optimizing-compiler-thread.cc
diff --git a/src/optimizing-compiler-thread.cc b/src/optimizing-compiler-thread.cc
index fbc4f05997b32768df14b4c532d47c5ae95f95a9..32a7f971401b65e7521b94edccc114bf0c8ec0d6 100644
--- a/src/optimizing-compiler-thread.cc
+++ b/src/optimizing-compiler-thread.cc
@@ -370,8 +370,13 @@ void OptimizingCompilerThread::AddToOsrBuffer(RecompileJob* job) {
 
 
 #ifdef DEBUG
+bool OptimizingCompilerThread::IsOptimizerThread(Isolate* isolate) {
+  return isolate->concurrent_recompilation_enabled() &&
+         isolate->optimizing_compiler_thread()->IsOptimizerThread();
+}
+
+
 bool OptimizingCompilerThread::IsOptimizerThread() {
-  if (!FLAG_concurrent_recompilation) return false;
   LockGuard<Mutex> lock_guard(&thread_id_mutex_);
   return ThreadId::Current().ToInteger() == thread_id_;
 }