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

src/execution.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 933 matching lines @@
   }
   if (stack_guard->IsInterrupted()) {
     stack_guard->Continue(INTERRUPT);
     return isolate->StackOverflow();
   }
   if (stack_guard->IsFullDeopt()) {
     stack_guard->Continue(FULL_DEOPT);
     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();
   }
   isolate->runtime_profiler()->OptimizeNow();
   return isolate->heap()->undefined_value();
 }
 
 
 } }  // namespace v8::internal