Remove usage of cr.js in chrome://credits

cr.js is deprecated in favor of its JS module equivalent (cr.m.js). In
the case of chrome://credits none of them are necessary though, since
|cr.isChromeOS| can be replaced with <if expr> preprocessing at build
time.

Bug: 1028829
Change-Id: I6db4a52b78533e82d5467be9721bf46669002acf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2658085


Commit-Queue: dpapad <dpapad@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848706}