Enable setting Accept-Language from the command-line

Today, the Accept-Language is constructed by concatenating any
policy-forced languages with the |intl.selected_languages| preference
stored in the user's profile.

We have received requests to allow specifying a different
Accept-Language via the command line, akin to how the --lang
command-line argument permits temporarily overriding the browser's UI
language.

This CL addresses the request by adding a --accept-lang command-line
argument that overrides the profile preference via the
CommandLinePrefStore. Any policy-forced languages will continue to be
prepended to the Accept-Language header value.

Bug: 1291310
Change-Id: I86672aad5c5079a673d9cdbad330b95a2ccbfb17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3418064
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Scott Little <sclittle@chromium.org>
Commit-Queue: Eric Lawrence <ericlaw@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#967559}