Revive the message aware message mutator.

This is adapted to use pre-recorded message files. It transforms an input message file into a mutated message file. There is also a generate utility to create well-formed but random messages out of thin air.

BUG=260848
R=aedla@chromium.org
TBR=kalman@chromium.org

Review URL: https://codereview.chromium.org/89893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242246 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242246