Originally the url test utils had UTF string conversion functions because we
didn't want the library to depend on STL (for embedders). We've not had this
limitation for a long time so the UTF16 <--> UTF8 conversion functions can be
deleted in favor of the ones in base.

The one remaining function which generates possibly-invalid UTF16 (by design)
is renamed and clarified for this purpose.

Review-Url: https://codereview.chromium.org/2469133002
Cr-Commit-Position: refs/heads/master@{#429688}