VM: Small clean up and const-ness fix for Thread/OSThread constants.

runtime/bin/thread_macos.cc

Index: runtime/bin/thread_macos.cc
diff --git a/runtime/bin/thread_macos.cc b/runtime/bin/thread_macos.cc
index be1b21af4ae43002a8217673d1377e27204de7ee..28c747d65073ea25ffe9f1eca71904d54278a779 100644
--- a/runtime/bin/thread_macos.cc
+++ b/runtime/bin/thread_macos.cc
@@ -107,8 +107,9 @@ int Thread::Start(ThreadStartFunction function, uword parameter) {
 }
 
 
-ThreadLocalKey Thread::kUnsetThreadLocalKey = static_cast<pthread_key_t>(-1);
-ThreadId Thread::kInvalidThreadId = reinterpret_cast<ThreadId>(NULL);
+const ThreadLocalKey Thread::kUnsetThreadLocalKey =
+    static_cast<pthread_key_t>(-1);
+const ThreadId Thread::kInvalidThreadId = reinterpret_cast<ThreadId>(NULL);
 
 ThreadLocalKey Thread::CreateThreadLocal() {
   pthread_key_t key = kUnsetThreadLocalKey;