base: Constantify thread_name() getter accessor.

TEST=base_unittests
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Review URL: https://chromiumcodereview.appspot.com/11293086

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@165904 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/tracked_objects.h b/base/tracked_objects.h
index 13e1f75..a4e971d 100644
--- a/base/tracked_objects.h
+++ b/base/tracked_objects.h
@@ -421,7 +421,7 @@
       const TrackedTime& start_of_run,
       const TrackedTime& end_of_run);
 
-  const std::string thread_name() const;
+  const std::string& thread_name() const { return thread_name_; }
 
   // Hack: asynchronously clear all birth counts and death tallies data values
   // in all ThreadData instances.  The numerical (zeroing) part is done without