Follow up to replacing v8String calls with v8SetReturnValueString.

This should've been done in https://codereview.chromium.org/18336004/ and was
somehow missed by https://chromiumcodereview.appspot.com/18106005/ .

BUG=236290
R=ch.dumez@sisa.samsung.com, haraken@chromium.org

Review URL: https://codereview.chromium.org/19278012

git-svn-id: svn://svn.chromium.org/blink/trunk@154296 bbb929c8-8fbe-4397-9dbb-9b2b20218538