base: Constantify thread_name() getter accessor.

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index 645e67c81b970aee4bae2e08608990111ac59707..cf1a86da4d39e374c066c7c090dd14b8c17ae7d2 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -131,7 +131,7 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   }
 
   // Returns the name of this thread (for display in debugger too).
-  const std::string &thread_name() { return name_; }
+  const std::string& thread_name() const { return name_; }
 
   // The native thread handle.
   PlatformThreadHandle thread_handle() { return thread_; }