A subsequent change will add MachMessageWithDeadline(), a mach_msg()
wrapper. Conceptually, it makes sense to include that function in this
file family. Since this file family now contains a mach_msg() wrapper,
it makes sense to rename it mach_message and lose the _util suffix.
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/772133004