If validation layers are requested by default, also enabled them in
VulkanExternalHelper.

Bug: angleproject:3289
Change-Id: Ic75089d38992be3b4745ce0309bf4ce6b8a702b1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2195683Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>