about_os_credits.html has been edited a couple of times to change the

about_os_credits.html has been edited a couple of times to change the
doctype and the default webUI font. However, they're not actually used
in CrOS under normal circumstances. A real fix/change in styles and layout
should go to chromite/licensing/about_credits.tmpl.

This CL makes it clear that it's a place holder.

Also, delist the file from the Android webview whitelist.

BUG=486166
TEST=None
R=dgarrett@chromium.org, torne@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/1138683003

Cr-Commit-Position: refs/heads/master@{#331029}