Use base::span<> in text break iterators

This case was selected to illustrate the utility of the new
WTF::String::Span8() and Span16() methods, and of spans in general.

Bug: 955067
Change-Id: I5108c38bba6dde93429f317ab7a6d9a028d3c739
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575866
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653035}
11 files changed