angle_enable_vulkan_validation_layers is false on is_mac,
so it looks like they were built by accident. Guard the dep
on $angle_root/src/common/vulkan:vulkan_validation_layers by
angle_enable_vulkan_validation_layers instead of angle_enable_vulkan,
and only define the target if angle_enable_vulkan_validation_layers
is true (because src/common/vulkan/BUILD.gn gets loaded due to deps
on other targets in it, and when a .gn file is loaded all the
targets in it get defined).

Since vulkan_validation_layers doesn't currently build on
arm macs, this happens to fix a build problem there, but it
also makes the build on intel macs a bit smaller.

This sidesteps comment 13 on the bug. If we ever want to use
the validation layers on mac, we'll have to do a real fix then.

Bug: chromium:1098741
Change-Id: I8164ff13de98d34ed875c6e6d0550eae64b2d0dd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2289042
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>