Call WTFString::split(UChar, Vector&) when possible

Use WTFString::split(UChar, Vector&) when possible instead of
Use WTFString::split(const String&, Vector&) to avoid constructing String
objects unnecessarily.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/359903002

git-svn-id: svn://svn.chromium.org/blink/trunk@177312 bbb929c8-8fbe-4397-9dbb-9b2b20218538