Use icu::BreakIterator instead of the ubrk_* api. It makes the code cleaner.

BUG=232922

Review URL: https://codereview.chromium.org/23618052

git-svn-id: svn://svn.chromium.org/blink/trunk@159441 bbb929c8-8fbe-4397-9dbb-9b2b20218538