[CSS Shapes] Omit polygon default fill-rule

Omit the default value (nonzero) when serializing the fill-rule. Adjust the tests to
this behavior.

BUG=350148

Review URL: https://codereview.chromium.org/191353002

git-svn-id: svn://svn.chromium.org/blink/trunk@169035 bbb929c8-8fbe-4397-9dbb-9b2b20218538