CSS Tokenizer: Implement <url-token> and <bad-url-token>

This patch implements <url-token> and <bad-url-token> from css-syntax.
I've done this slightly differently to the spec, dropping whitespace
tokens in e.g. url( "hello"). These whitespace tokens wouldn't be used
and the logic is a bit simpler this way.

Note that the tokenization logic has changed between the candidate
rec and the latest editor's draft; I've implemented the latest version.

http://dev.w3.org/csswg/css-syntax/#consume-url-token

BUG=424988

Review URL: https://codereview.chromium.org/659913004

git-svn-id: svn://svn.chromium.org/blink/trunk@184482 bbb929c8-8fbe-4397-9dbb-9b2b20218538