This makes ES3_VULKAN() pass the instantiability test, allowing it to be
specified in end2end tests.

Bug: angleproject:2950
Change-Id: Ife70a22fb8193f9eebe64bec491a24b47bc76939
Reviewed-on: https://chromium-review.googlesource.com/c/1325729Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>