Vulkan: Enable validation layers on request. Also adds the build files for the Vulkan layers. The layers are enabled by default for the tests. BUG=angleproject:1319 Change-Id: I0b442b36312a1299a932922e1c4e39f00801de49 Reviewed-on: https://chromium-review.googlesource.com/367751 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>