[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