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This change modifies base::string16 to be an alias for std::u16string
on non-Windows platforms. This allows the removal of a custom
std::char_traits specialization for uint16_t, and now also allows
the creation of base::string16s from char literals on all platforms,
removing most of the need for helpers like base::ASCIIToUTF16.

Lastly, this change also updates blink::WebUChar, ensuring it continues
to be the exact same type as base::char16.

Bug: 911896
Change-Id: Ifef35543e10cfee2b25f11984be289553532626e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586354Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841439}