Using mtime is fundamentally flawed when working with git. Replace
these flaky checks with hash comparisons. The hashes are stored in an
autogenerated json file that will be stored in the repository.

This makes the run_code_generation script robust against any and all
input changes. It also removes the need to track script outputs as
dependencies.

Bug: angleproject:2697
Change-Id: I60f2a87a8680b1f775ad678b05112f5b16c7dde7
Reviewed-on: https://chromium-review.googlesource.com/1120159
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>