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This change prepares the non-Windows part of the codebase for the
base::string16 switch to std::u16string. This is done by adding explicit
casts where current code relies on base::char16 being int16_t.

Furthermore, WTF::String, WTF::StringView and WTF::AtomicString are
modified to only expose a constructor from const char16_t* when char16_t
and icu's UChar are different types.

Lastly, this change removes a couple banned usages of NoCopy creation of
(Core)Foundation type. Apart from that this change does not change any
functionality.

TBR=jbroman,mmenke,jamiewalch

Bug: 911896
Change-Id: Ie208e7a5e7f00097d0a07eb4fd9197284cd8c482
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596261Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838264}