Mojo: Remove MessageInTransit::View's message_size_ member.

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/183513009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254311 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254311