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This change implements wide string overloads to string_split.h
and string_util.h.
Since these functions are not supposed to be used by Cross-Platform
code it is guarded by correpsonding #ifdef guards. Furthermore,
this change performs some minor code clean-ups and performance
improvements.

Bug: 911896
Change-Id: I579b55a398307c890b30a027699b74a9afb1a77e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823851
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703617}