Factor out (and optimize) MessageInTransit.

This is mainly so that MessageInTransit has room for a header (for use in other
code), but while at it I optimized it a bit. The 10- and 100-byte
SystemPerftest.MessagePipe_WriteAndRead performance tests go from ~2.3 million
iterations/second to ~2.6; in perf, |free()| is no longer the top hotspot in
SystemPerftest.MessagePipe_WriteAndRead (all sizes) -- instead, |memcpy()| is,
as you'd hope.

R=darin

Review URL: https://codereview.chromium.org/25443011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226839 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226839