Cleanup usage of String::append

Cleanup usage of String::append in some parts of the Blink source.

BUG=268281

Review URL: https://codereview.chromium.org/145033003

git-svn-id: svn://svn.chromium.org/blink/trunk@166539 bbb929c8-8fbe-4397-9dbb-9b2b20218538