In CSSFilterValue::customCSSText we could replace
string building with StringBuilder for a better peformance.

BUG=

Review URL: https://codereview.chromium.org/140883003

git-svn-id: svn://svn.chromium.org/blink/trunk@165982 bbb929c8-8fbe-4397-9dbb-9b2b20218538