Vulkan: Allow for no .git directory in build. 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/854255 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>