Thread pool tidying.

Make fields const. Fix memory leak in gtest caused by not allowing thread pool
to empty tasks.

Change-Id: I32b20dee17eab03443c4bd1f3cc102c4409d1ab9
diff --git a/src/thread_pool.h b/src/thread_pool.h
index e97d7d6..18af97d 100644
--- a/src/thread_pool.h
+++ b/src/thread_pool.h
@@ -50,7 +50,7 @@
   static void* Callback(void* arg) LOCKS_EXCLUDED(Locks::mutator_lock_);
   virtual void Run();
 
-  ThreadPool* thread_pool_;
+  ThreadPool* const thread_pool_;
   const std::string name_;
   const size_t stack_size_;
   pthread_t pthread_;