This will allow us to avoid loading the build file for
//third_party/vulkan-validation-layers on Fuchsia, since we don't want to
build the validation layers on that platform.

BUG=angleproject:2475
TEST=angle_end2end_tests on Fuchsia (with full series)

Change-Id: Ifeb67125f5fcebf2b30ae18b760667ba7c56e4d5
Reviewed-on: https://chromium-review.googlesource.com/c/1456479
Commit-Queue: Michael Spang <spang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>