Added system thread manager class.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index b0142bff0d9e536c259b769d6dc2ce17b1a8518a..500c691eafe7d6da7d43ee5bd5b6fcfddc209f29 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -287,6 +287,20 @@ class ThreadLocalTop BASE_EMBEDDED {
 };
 
 
+class SystemThreadManager {
+ public:
+  enum ParallelSystemComponent {
+    PARALLEL_SWEEPING,
+    CONCURRENT_SWEEPING,
+    PARALLEL_MARKING
+  };
+
+  static int NumberOfParallelSystemThreads(ParallelSystemComponent type);
+
+  static const int kMaxThreads = 4;
+};
+
+
 #ifdef ENABLE_DEBUGGER_SUPPORT
 
 #define ISOLATE_DEBUGGER_INIT_LIST(V)                                          \