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

runtime/bin/thread_linux.cc

Index: runtime/bin/thread_linux.cc
diff --git a/runtime/bin/thread_linux.cc b/runtime/bin/thread_linux.cc
index dc07de4bddf6a77019ff4013238532b37040c1a5..1e01a7788acd7d48356802d2838a0b0d46d012eb 100644
--- a/runtime/bin/thread_linux.cc
+++ b/runtime/bin/thread_linux.cc
@@ -115,8 +115,9 @@ int Thread::Start(ThreadStartFunction function, uword parameter) {
 }
 
 
-ThreadLocalKey Thread::kUnsetThreadLocalKey = static_cast<pthread_key_t>(-1);
-ThreadId Thread::kInvalidThreadId = static_cast<ThreadId>(0);
+const ThreadLocalKey Thread::kUnsetThreadLocalKey =
+    static_cast<pthread_key_t>(-1);
+const ThreadId Thread::kInvalidThreadId = static_cast<ThreadId>(0);
 
 ThreadLocalKey Thread::CreateThreadLocal() {
   pthread_key_t key = kUnsetThreadLocalKey;