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This change moves the Windows only code in base/strings/{strcat,
string_util, string_split}.h to their own _win.h headers, and introduces
appropriate _internal.h headers to be able to share the templated code
between Windows and cross-platform code.

This change is purely organizational, and there is no functionality
added or removed.

Bug: 911896
Change-Id: I3d2eae1e665cfcf48e727d6d71ca1975c441a370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228622
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775075}