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Since both Java and JavaScript uses UTF16 strings, makes it more
compatible to use base::string16 in C++.

Per comment from http://crrev/c/1719453.

Bug: 918065
Change-Id: Ie979c0e400604ec2230bd7cb8319dc74f39616e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1750093
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686511}