Mojo: Factor MessageInTransit secondary buffer stuff out into a separate class.

Also write a big comment explaining what I'm going to do.

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Review URL: https://codereview.chromium.org/265753006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268032 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/mojo/mojo.gyp b/mojo/mojo.gyp
index fdd6bf86..70c835c 100644
--- a/mojo/mojo.gyp
+++ b/mojo/mojo.gyp
@@ -192,6 +192,8 @@
         'system/shared_buffer_dispatcher.h',
         'system/simple_dispatcher.cc',
         'system/simple_dispatcher.h',
+        'system/transport_data.cc',
+        'system/transport_data.h',
         'system/waiter.cc',
         'system/waiter.h',
         'system/waiter_list.cc',