run_code_generation: Compare hashes instead of mtime. 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>