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This change removes the deprecated base::c16memcpy, base::c16len and
base::c16memcpy, updates the comment ontop of string16.h to make clear
it is only a type alias nowadays and removes obsolete tests or moves
them in more appropriate locations.

Bug: 911896
Change-Id: I5f66b8db5d81c962e040c79eed844136fd570ee2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2642270Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846008}