This adds build_overrides/spirv_tools.gni to tell it where to find its
dependencies.

BUG=chromium:870747
Change-Id: Id1d9fb836e74597860a4411df0a4fd0651840f57
Reviewed-on: https://chromium-review.googlesource.com/1162240Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>