Removed const from overrides.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/4b4eed074e848347f5a5b07d4d4b3fd2f6e13820

commit 4b4eed074e848347f5a5b07d4d4b3fd2f6e13820
Author: Lucas Furukawa Gadani <lfg@chromium.org>
Date:   Tue Jun 4 23:12:04 2019 +0000

    Remove const interfaces from content/public.

    Tbr: boliu@chromium.org, dvadym@chromium.org, bengr@chromium.org
    Bug: 908139