We want to be able to build without validation layers to
make it easier to use in other environments (e.g. Android)
When angle_enable_vulkan is true and
angle_enable_vulkan_validation_layers is true, build the
Vulkan validation layers.

Bug: angleproject:2489

Change-Id: I266ad8f757c5992021f249217f2be58f0148fcbf
Reviewed-on: https://chromium-review.googlesource.com/1005558Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>