CSS Tokenizer: Consume a single whitespace after escapes if present

The css-syntax spec says to consume a single whitespace after a
hex-digit escape if present.

http://dev.w3.org/csswg/css-syntax/#consume-escaped-code-point

BUG=424988

Review URL: https://codereview.chromium.org/649493003

git-svn-id: svn://svn.chromium.org/blink/trunk@183989 bbb929c8-8fbe-4397-9dbb-9b2b20218538