Drive-by utf8 serialization exposed on StringUtil for inspector protocol.

Bug: chromium:929862
Change-Id: I930cd43fef9038471908280f15e65a4edec5c6d1
Reviewed-on: https://chromium-review.googlesource.com/c/1468702
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59542}