Move the implementation from CdmPromiseTemplate into a new templated
class CdmCallbackPromise class. CdmResultPromise can now override
resolve() and reject() without having to be concerned with the
callbacks.

Also convert the templates into variadic templates to avoid the
specialization for <void> (now just <>).

BUG=358271
TEST=existing EME tests still pass

Review URL: https://codereview.chromium.org/604283003

Cr-Commit-Position: refs/heads/master@{#298554}