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

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 671d06fac85e606b441aee81d1b0e90824108400..1821f71ee6da29a9fb74b04c219eecbb75f04ac4 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -9499,7 +9499,7 @@ void JSFunction::MarkForConcurrentRecompilation() {
   ASSERT(!IsOptimized());
   ASSERT(shared()->allows_lazy_compilation() || code()->optimizable());
   ASSERT(!shared()->is_generator());
-  ASSERT(FLAG_concurrent_recompilation);
+  ASSERT(GetIsolate()->concurrent_recompilation_enabled());
   if (FLAG_trace_concurrent_recompilation) {
     PrintF("  ** Marking ");
     PrintName();
@@ -9517,7 +9517,7 @@ void JSFunction::MarkInRecompileQueue() {
   ASSERT(!GetIsolate()->DebuggerHasBreakPoints());
   ASSERT(IsMarkedForConcurrentRecompilation() && !IsOptimized());
   ASSERT(shared()->allows_lazy_compilation() || code()->optimizable());
-  ASSERT(FLAG_concurrent_recompilation);
+  ASSERT(GetIsolate()->concurrent_recompilation_enabled());
   if (FLAG_trace_concurrent_recompilation) {
     PrintF("  ** Queueing ");
     PrintName();