Vulkan natively supports non power-of-two textures so indicate
that support for front-end.

Bug: angle-project:3239
Test: angle_deqp_gles2_tests --gtest_filter=dEQP.GLES2/functional_texture_completeness_2d_npot_t_repeat --use-angle=vulkan
Change-Id: Id9058b6e9afd1d43e5d74612f808ce39beafd35f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1561963
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>