[CSS Shapes] Adjust inset sizing syntax to the latest specification

According to the latest CSS Shapes specification [1], the width arguments of inset should
follow the syntax of the margin shorthand, which let us set all four insets with one, two
or four values. This patch updates the behavior and updates the affected tests.

[1] http://dev.w3.org/csswg/css-shapes/#funcdef-inset

BUG=339009

Review URL: https://codereview.chromium.org/149063007

git-svn-id: svn://svn.chromium.org/blink/trunk@166256 bbb929c8-8fbe-4397-9dbb-9b2b20218538