Remove 'String::append' and move to 'StringBuilder::append' in core/rendering

BUG=268281

Review URL: https://codereview.chromium.org/23731013

Review URL: https://chromiumcodereview.appspot.com/23731013

git-svn-id: svn://svn.chromium.org/blink/trunk@158319 bbb929c8-8fbe-4397-9dbb-9b2b20218538