Efficiently determining when to build about_credits.html is hard.
Currently it is marked as dirty anytime the root .ninja file is
modified, so after every "gn gen" or modification of any BUILD.gn file
it gets rewritten, triggering 106 steps on Windows, 58 steps on Linux.

This change just makes the script smart enough to not write the file if
its contents have not changed. Thus, 105 of the 106 steps on Windows,
including linking of chrome.exe, can be skipped.

In addition to slightly improving build times this change makes it more
obvious what is going on. It makes it (fairly) clear that
about_credits.html is being regenerated but has not actually changed.

R=agrieve@chromium.org
BUG=692601

Review-Url: https://codereview.chromium.org/2780983004
Cr-Commit-Position: refs/heads/master@{#460807}