Add comment against usage of base::Thread.

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index 01f7d8e250c7ef6ca241618d3ceb36fbb790d69e..2db69c2148019ad3c666d001524344dc45ce1ce2 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -28,6 +28,9 @@ namespace base {
 class MessagePump;
 class RunLoop;
 
+// IMPORTANT: Instead of creating a base::Thread, consider using
+// base::Create(Sequenced|SingleTreaded)TaskRunnerWithTraits().
+//
 // A simple thread abstraction that establishes a MessageLoop on a new thread.
 // The consumer uses the MessageLoop of the thread to cause code to execute on
 // the thread.  When this object is destroyed the thread is terminated.  All