Update SPIRV-Tools build

This CL updates the SPIRV-Tools DEPS entry. This requires adding a
SPIRV-Headers third_party dependency as the headers are no-longer part
of SPIRV-Tools.

The BUILD.gn file for SPIRV-Tools is added directly to the SPIRV-Tools
repo and so third_party/SPIRV-Tools/BUILD.gn can be removed. The
SPIRV-Tools build contains fuzzing targets which are enabled with this
change.

Change-Id: I9181a1b50c1f08bbe4e68dff0e00b927d297745f
Reviewed-on: https://chromium-review.googlesource.com/1142028
Commit-Queue: dsinclair <[email protected]>
Reviewed-by: Antoine Labour <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Cr-Commit-Position: refs/heads/master@{#579914}
8 files changed