CSS Tokenizer: Idents can start with --

The latest versions of css-syntax allow idents to start with --.
http://dev.w3.org/csswg/css-syntax/#would-start-an-identifier

BUG=424988

Review URL: https://codereview.chromium.org/681683002

git-svn-id: svn://svn.chromium.org/blink/trunk@184445 bbb929c8-8fbe-4397-9dbb-9b2b20218538