[net] Prepare //net/dns for the base::string16 switch

This change prepares //net/dns for the upcoming switch of base::string16
to std::u16string.

Bug: 911896
Change-Id: If9d3dd0af5f4007698bff02a0e453e63257d7d0c
Reviewed-on: https://chromium-review.googlesource.com/c/1450196
Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629328}
2 files changed