Fixed MediaQueryTokenizer escape support

MediaQueryTokenizer handling of escapes was faulty, causing media queries with escaped code points to be considered invalid.
This CL fixes that.

BUG=358833

Review URL: https://codereview.chromium.org/221383003

git-svn-id: svn://svn.chromium.org/blink/trunk@170704 bbb929c8-8fbe-4397-9dbb-9b2b20218538