Treat marker like a real shorthand for initial and inherit

Setting the SVG CSS property "marker" to initial or inherit would
hit an ASSERT in StyleBuilder::applyProperty().
Because marker is not handled by shorthandForProperty() despite it being
a shorthand property it leaks into the style declaration without
expansion due to special case handling of initial and inherit in
CSSPropertyParser::parseValue().

Review URL: https://codereview.chromium.org/339713008

git-svn-id: svn://svn.chromium.org/blink/trunk@184427 bbb929c8-8fbe-4397-9dbb-9b2b20218538