Replace "nested message loop" with "nested run loop" in comments.

ppapi/cpp/message_loop.h

Index: ppapi/cpp/message_loop.h
diff --git a/ppapi/cpp/message_loop.h b/ppapi/cpp/message_loop.h
index 4df2dd72e36fcebd03ef1c609353cc26e79eeb3d..c97fdf520aac67be613248dd70569c99a7f1f841 100644
--- a/ppapi/cpp/message_loop.h
+++ b/ppapi/cpp/message_loop.h
@@ -184,7 +184,7 @@ class MessageLoop : public Resource {
   /// The message loop identified by the argument must have been previously
   /// successfully attached to the current thread.
   ///
-  /// You may not run nested message loops. Since the main thread has an
+  /// You may not run nested run loops. Since the main thread has an
   /// implicit message loop that the system runs, you may not call Run on the
   /// main thread.
   ///