UniversalMachExcServer: eliminate multiple implementation inheritance

UniversalMachExcServer: eliminate multiple implementation inheritance.

UniversalMachExcServer provided both an interface and an implementation,
contrary to the other classes in the exc_server_variants family. This
was mostly done for reasons of economy in an already-large class family.
Unfortunately, this decision meant that it was impossible for other code
to use UniversalMachExcServer, which required that CatchMachException()
be implemented, and also extend another class without violating the
style guide’s prohibition of multiple implementation inheritance. This
became a problem in a lot of test code, which extended MachMultiprocess
and UniversalMachExcServer.

UniversalMachExcServer is now given its own nested Interface class,
which is a pure interface. All users of UniversalMachExcServer are
changed from “is-a” UniversalMachExcServer to “has-a”
UniversalMachExcServer and “is-a” UniversalMachExcServer::Interface.

TEST=client_test, snapshot_test, util_test
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/821ed8fe0fda1c024b08243a71b4676ebce00e9e

[Mark Mentovai, 2014-12-03 21:36:08]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Robert Sesek, 2014-12-04 15:16:55]

LGTM.

I'm glad there's tests for this, because it'd be otherwise annoying to check
that all the arguments still do the right thing.

[Mark Mentovai, 2014-12-04 15:18:29]

Committed patchset #1 (id:1) manually as
821ed8fe0fda1c024b08243a71b4676ebce00e9e (presubmit successful).