Base: Don't overwrite the thread id when ThreadMain exits. There's

no need to do that, and it makes debugging easier to have the value.

BUG=127931
TEST=none
Review URL: https://chromiumcodereview.appspot.com/10388202

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@138014 0039d316-1c4b-4281-b951-d872f2087c98

base/threading/thread.cc

-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -171,7 +171,6 @@ void Thread::ThreadMain() {
     // We can't receive messages anymore.
     message_loop_ = NULL;
   }
-  thread_id_ = kInvalidThreadId;
 }
 
 }  // namespace base