The SPIRV-Tools 'external revision generate' script is coded such that
it assumes a .git folder is present. This causes a problem for the
tarball build. We can fix this by using the same tooling we use for
ANGLE's commit id script to check if the .git directory is present
before running the generator. If it is missing, we insert a hard-coded
dummy header. Also use the 'DEPS' file as a placeholder for .git/HEAD
to check if the current revision needs updating.

Bug: chromium:799620
Bug: angleproject:2237
Change-Id: Icea8e9c66f1600df7dca2aaa45fe449f687f5b55
Reviewed-on: https://chromium-review.googlesource.com/854255Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>