[GN] Don't rewrite files with the same contents

BUG=

Review URL: https://codereview.chromium.org/1656253003

Cr-Original-Commit-Position: refs/heads/master@{#373544}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f8ea5cceefcedd4a01935d5ac4d2ba71e23ac13e
6 files changed
tree: ec0493358b5d97292417157458d4de50579ddeef
  1. tools/