Remove XML and CSS parser externs

This allows tools like scythe to match up the cssyyparse and xpathyyparse
symbols with their consumers

BUG=335193

Review URL: https://codereview.chromium.org/130663003

git-svn-id: svn://svn.chromium.org/blink/trunk@165247 bbb929c8-8fbe-4397-9dbb-9b2b20218538