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Spellcheck languages that have Windows platform support but not
Hunspell support (Arabic, Finnish, ...) cannot be disabled or forced
enabled by enterprise policy. This is because of the use of
spellcheck::GetCorrespondingSpellCheckLanguage in
SpellcheckLanguageBlacklistPolicyHandler::SortBlacklistedLanguages and
SpellcheckLanguagePolicyHandler::SortForcedLanguages, a method that
only recognizes Hunspell languages.

The fix is to instead check if a language with policy being enforced is
on the list of supported accept languages, as these are what are used
to populate the language list on the language settings page. Note that
in the upcoming implementation of support for non-Hunspell languages,
we will only allow Windows platform spellchecking for these accept
languages (if they have a Windows spellcheck dictionary).

Instead of just searching for the accept language within the tag, the
algorithm first tries for an exact match (so "en-CA" would be found),
then searches for an exact match for the language without locale
info, that is, what's to the left of the hyphen in a language tag
(so "ar" would be found for "ar-SA").

Also adds unit test coverage for the spellcheck policy handlers, which
previously had none.

Note that there is an interplay between forced and blocked languages,
as well as the spellcheck-enabled policy:

  1) If a language is both forced and blocked, forced wins.
  2) If spellcheck is disabled by policy, the forced/blocked languages
     policy has no effect.

Test cases cover this interplay.

Bug: 1056850
Change-Id: Iec54a851d38f00a7aa7597449f7acc30f3dd2de3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2080450Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Bruce Long <brlong@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#757107}