Enables the user to select multiple languages for spellchecking (UI)

This patch enables the user to select multiple languages for
spellchecking using the right click menu, as well as through the
chrome://settings/languages. It can be enabled through a command
line flag "--enable-multilingual-spellchecker". It also adds the
option to enable or disable this flag using the chrome://flags page.

TEST=*SpellcheckService*
TEST=*Multilanguage*

Original patch from Klemen Forstnerič (klemen.forstneric@gmail.com).
BUG=5102

Review URL: https://codereview.chromium.org/1156473007

Cr-Commit-Position: refs/heads/master@{#338174}
14 files changed