Only allow static strings as ProcessThread names.

Review URL: https://codereview.webrtc.org/1336293002

Cr-Commit-Position: refs/heads/master@{#9932}
diff --git a/webrtc/modules/utility/source/process_thread_impl.h b/webrtc/modules/utility/source/process_thread_impl.h
index 53b5d59..5101ea9 100644
--- a/webrtc/modules/utility/source/process_thread_impl.h
+++ b/webrtc/modules/utility/source/process_thread_impl.h
@@ -76,7 +76,7 @@
   // TODO(tommi): Support delayed tasks.
   std::queue<ProcessTask*> queue_;
   bool stop_;
-  std::string thread_name_;
+  const char* thread_name_;
 };
 
 }  // namespace webrtc