Add Dawn's dependencies shaderc and glslang, and roll SPIRV. Update the
BUILD.gn files to match upstream changes.
Bug: skia:9939, chromium:1064563
Change-Id: Ie9720806efe823312da10f7911688f12cad0713e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/278770
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Weston Tracey <westont@google.com>
