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

src/execution.cc

Index: src/execution.cc
diff --git a/src/execution.cc b/src/execution.cc
index 979641a9de53c1cd7ebfce9be9bf069699b0fe9f..8febbbf2a791deb7ae1e8395a332c7c79dc49a7c 100644
--- a/src/execution.cc
+++ b/src/execution.cc
@@ -951,7 +951,7 @@ MaybeObject* Execution::HandleStackGuardInterrupt(Isolate* isolate) {
     Deoptimizer::DeoptimizeAll(isolate);
   }
   if (stack_guard->IsInstallCodeRequest()) {
-    ASSERT(FLAG_concurrent_recompilation);
+    ASSERT(isolate->concurrent_recompilation_enabled());
     stack_guard->Continue(INSTALL_CODE);
     isolate->optimizing_compiler_thread()->InstallOptimizedFunctions();
   }