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

src/compiler.h

Index: src/compiler.h
diff --git a/src/compiler.h b/src/compiler.h
index 5ebcfd97cb1c40457f6a43c5b4c14b256a782da9..080907e390ca3e3b14adebfb02b90a61c1867e91 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -302,12 +302,12 @@ class CompilationInfo {
   }
 
   void AbortDueToDependencyChange() {
-    ASSERT(!isolate()->optimizing_compiler_thread()->IsOptimizerThread());
+    ASSERT(!OptimizingCompilerThread::IsOptimizerThread(isolate()));
     abort_due_to_dependency_ = true;
   }
 
   bool HasAbortedDueToDependencyChange() {
-    ASSERT(!isolate()->optimizing_compiler_thread()->IsOptimizerThread());
+    ASSERT(!OptimizingCompilerThread::IsOptimizerThread(isolate()));
     return abort_due_to_dependency_;
   }