We can use GetAs() to retrieve the corresponding API object.

BUG=angleproject:963

Change-Id: I472e13664e45f9f05621dea3d28135d4e833fb3d
Reviewed-on: https://chromium-review.googlesource.com/263485Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>