grit: Use new --write-only-new flag.

For the file I'm testing with (c/b/resources/about_stats.html),
a rebuild runs 101 edges and compiles a bunch of cc files and takes 40s.
This reduces build times from 40s to 9.6s and reduces the number
of edges to 37.

BUG=439182
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/740463005

Cr-Commit-Position: refs/heads/master@{#306945}
4 files changed