Added task kind field to thread JSON output.

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index 01f16982527a05554a1863f5081ebfa59461f4ab..65bbdf042be0d38aa6a822c9eb94f65fe27a296a 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -161,6 +161,9 @@ class Thread : public BaseThread {
     kMarkerTask = 0x8,
     kFinalizerTask = 0x10,
   };
+  // Converts a TaskKind to its corresponding C-String name.
+  static const char* TaskKindToCString(TaskKind kind);
+
   ~Thread();
 
   // The currently executing thread, or NULL if not yet initialized.