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This change registers the PluralStringHandler as a MessageHandler for
the SettingsUI and adds PluralStringProxy as a thin JavaScript wrapper
around it.

Bug: 1047726
Change-Id: I4c53eed726711635adb29e63435dd9da697d0635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2090425Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747854}