[spellcheck] Don't spellcheck in removed language.

If you remove from chrome://settings/languages a language that has "Use
this language for spellchecking" selected, then this language will
continue to be used for spellchecking. This patch fixes the issue by
listening to changes in accept languages and disabling spellcheck for
any language that is no longer in accept languages.

BUG=584519

Review URL: https://codereview.chromium.org/1673783003

Cr-Commit-Position: refs/heads/master@{#374485}
3 files changed