Using reflection helpers, we can avoid having to pass the return
type to the GetImpl method.

BUG=angleproject:2200

Change-Id: Id160fbdf0ea8b3de2f219d3cc92394c678189a48
Reviewed-on: https://chromium-review.googlesource.com/742371
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>