Fix name in StringBuilder

We merged a slightly out-of-date version of
http://trac.webkit.org/changeset/152306 that missed a last-minute name change.

R=esprehn
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/19116002

git-svn-id: svn://svn.chromium.org/blink/trunk@154151 bbb929c8-8fbe-4397-9dbb-9b2b20218538