The implementations for the exc and mach_exc subsystems were nearly
identical, and were a good target for templatization. The existing
split between exc and mach_exc was a good candidate for unification
based on CompositeMachMessageServer instead of the custom unification
previously done in UniversalMachMessageServer.
TEST=util_test ExcServerVariants.*
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/766193006