Mojo: Minor cleanup (of message_in_transit.cc and message_pipe.*).

mojo/system/message_in_transit.h

Index: mojo/system/message_in_transit.h
diff --git a/mojo/system/message_in_transit.h b/mojo/system/message_in_transit.h
index 9e578afddfa466161cbfe6566274dd059eb84749..4018dd6c08e6126074846eb0506deb8dfe67e96b 100644
--- a/mojo/system/message_in_transit.h
+++ b/mojo/system/message_in_transit.h
@@ -242,8 +242,8 @@ class MOJO_SYSTEM_IMPL_EXPORT MessageInTransit {
   // To allow us to make assertions about |Header| in the .cc file.
   struct PrivateStructForCompileAsserts;
 
-  // "Header" for the data. Must be a multiple of |kMessageAlignment| bytes in
-  // size. Must be POD.
+  // Header for the data (main buffer). Must be a multiple of
+  // |kMessageAlignment| bytes in size. Must be POD.
   struct Header {
     // Total size of the message, including the header, the message data
     // ("bytes") including padding (to make it a multiple of |kMessageAlignment|