Remove UTF string conversion functions from the global namespace.

BUG=330556
TEST=no change
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/102993018

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f729d7a55a06bb521176d8bc354c8d88b452e067