Replace StringBuilder.append(String::number()) to appendNumber.

Corresponding patch in WebKit: bug 122382.

R=abarth@chromium.org, jochen@chromium.org, erik.corry@gmail.com

Review URL: https://codereview.chromium.org/582663002

git-svn-id: svn://svn.chromium.org/blink/trunk@182241 bbb929c8-8fbe-4397-9dbb-9b2b20218538