Remove 'String::append' from some of the blink source.

Replaced with StringBuilder whereever possible for (marginally) better performance.

BUG=268281

Review URL: https://codereview.chromium.org/124943003

git-svn-id: svn://svn.chromium.org/blink/trunk@164851 bbb929c8-8fbe-4397-9dbb-9b2b20218538