This change allows us to download a copy of the standalone glslang
validator during gclient runhooks. This decouples run_code_generation
from the version of glslang stored in source tree. This prevents the
error where a developer would run into a code generation conflict due
to a change in version of glslang.

Currently only the Windows version is stored in the tree. A follow-up
CL will add Linux. The validator will only initially support Windows
or Linux.

Documentation for the script is located in tools/glslang/README.md.
This CL also updates the Vulkan shader generation script to use the
new binary for Windows.

Bug: angleproject:3333
Change-Id: Ia6146a283a2bcfdf84c42411d6b5d8bae136bf1b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553823
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>