Adapt StringBuilder's append and toString methods via StringUtil helper.

This is needed to insulate generated code from blink::protocol namespace
from naming changes that we plan to do in the Great Blink Rename (which
in particular will change wtf::StringBuilder::toString into ToString,
and similarily will rename reserveCapacity and append methods).

BUG=683447
R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2655203003 .
3 files changed