lastHyphenLocation shouldn't require bloatedCharacters

Rather than bloating characters to create a UChar* and passing that array to
Chromium, I've taught Chromium how to receive a WebString in
https://codereview.chromium.org/19646004. That means we can use substring here
without doing extra work.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19652005

git-svn-id: svn://svn.chromium.org/blink/trunk@154506 bbb929c8-8fbe-4397-9dbb-9b2b20218538