Add a script to GRIT to generate predetermined resource ids file.

This script will be used to produce a resource ordering file with
the resources loaded during Chrome startup sequence that will be
fed back into GRIT via the mechanism implemented by:
  https://codereview.chromium.org/2690263004/

Also adds a README.md about how to use it.

BUG=692670

Review-Url: https://codereview.chromium.org/2696303003
Cr-Original-Commit-Position: refs/heads/master@{#451458}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fe16437ea76d582b4a931e219cd452756e2c6498
2 files changed