Since this was missing, the layer json files didn't get regenerated on
vulkan rolls, leading to stale generated json files, which in turn led
to incremental builds having different files in the swarming isolate
than full builds.

To make this type of bug harder to introduce, rewrite
generate_vulkan_layers_json.py a bit:

- pass in path to vulkan_core.h as an argument
- also pass in the input .json / .json.in files as arguments,
  so that the script re-runs if a .json or .json.in input is added
  or removed, and in the script verify that the passed-in list matches
  the glob() the script did previously (this verifies that the sources
  list in the .gn file is up-to-date with the state on disk)
- generate outputs list in gn from sources list, to make sure they're
  in sync
- use an expicit --icd flag instead of doing `'icd' in path`
- fail when failing to extract vk_version instead of silently using a
  default
- some minor python style fixes

Bug: chromium:910699,chromium:869348
Change-Id: I1e598f4566697a7f1ef56b040e52d0717f7ad075
Reviewed-on: https://chromium-review.googlesource.com/c/1358631Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>