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Because language -> voice pref is a DICTONARY type pref, this
change extends settings-dropdown-menu to work with dictionary
prefs given a new parameter, prefKey. When set, prefKey tells
the settings dropdown which key in the dictionary is associated
with the value it represents.

This is not yet hooked up to the Text-to-Speech controller. In
addition, we don't yet define default voices per language.

Bug: 823359
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I6b7ae800ed6f0230076da5d65114d305fc1dc64d
Reviewed-on: https://chromium-review.googlesource.com/1020456
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553911}