std::variant: Fixup .get() usage

This is just a refactor from the mapbox specific .get() interface to the
common std::variant ::get<>() interface.

Tested:
    Built and run through unit tests.

Change-Id: Ic0737f632e80c9dd5b73717ec33d31f245f107db
Signed-off-by: William A. Kennington III <wak@google.com>
4 files changed