(Rob, do you still think this is useful?)

From https://codereview.chromium.org/2795503002/ consider these ideas:

* a new Ozone entry OzonePlatform* EnsureInstance(const *char name) that makes a
specific platform pointer and returns it with the promise (noted in the
comments) that it does not alter the default instance or do anything that would
interfere with the existing platform where the string specifies the desired
platform by name.

* a new test platform (or set of options for configuring headless because that
was what it was for) that is appropriate for unit tests like this

See also TestOzonePlatform in platform_display_default_unittest.cc