Call characterAt rather than bloatedCharacters

There's no reason to call bloatedCharacters just to retrieve one UChar.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19675004

git-svn-id: svn://svn.chromium.org/blink/trunk@154448 bbb929c8-8fbe-4397-9dbb-9b2b20218538