Reject percentage values for outline-offset during CSS parsing

Percentages in outline-offset are not properly implemented, attempting
to use them will hit an assert in debug. There is no specification for
percentages in outline-offset [1]. Allowing percentages in the CSS
parser is likely an accidental change [2] and should be removed.

[1] http://www.w3.org/TR/css3-ui/#outline-offset
[2] http://src.chromium.org/viewvc/blink/trunk/WebCore/css/CSSParser.cpp?r1=66615&r2=66614&pathrev=66615

Review URL: https://codereview.chromium.org/66553009

git-svn-id: svn://svn.chromium.org/blink/trunk@161791 bbb929c8-8fbe-4397-9dbb-9b2b20218538