Drop StringBuilder from text rendering code.

This partially reverts r158319, though I honestly don't see how that
patch would have made performance worse. Regardless, this patch should
have strictly better performance by avoiding both String::append and
StringBuilder::append.

BUG=298926

Review URL: https://codereview.chromium.org/26240002

git-svn-id: svn://svn.chromium.org/blink/trunk@159308 bbb929c8-8fbe-4397-9dbb-9b2b20218538