[LSC] Replace base::string16 with std::u16string in //[f-z]*

This change replaces base::string16 with std::u16string in //[f-z]*

Reproduction steps:
$ git grep -lw 'base::string16' [f-z]* | \
      xargs sed -i 's/\bbase::string16\b/std::u16string/g'
$ git cl format

Bug: 1184339
Change-Id: I4e0ff2fd300c9fd9360dbf8f4b44e74f5f511797
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2752148


Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Owners-Override: Jan Wilken Dörrie <jdoerrie@chromium.org>
Owners-Override: danakj <danakj@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#862044}
GitOrigin-RevId: 739ccc21289257112c667e04a40d9a5a2db466bf