Updates our build files to be more consistent with Chromium.

Also rolls SPIRV-Tools to use newer test fixture syntax.
Note: requires an update to SPIRV-Tools to pass 'gn check'.

Bug: angleproject:3151
Change-Id: I8abddef396f84e5c58cdd185421f03fd6e2b6d58
Reviewed-on: https://chromium-review.googlesource.com/c/1470591Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@google.com>