[CSS Shapes] Support inset parsing

In this patch I added support for inset shape parsing for CSS Shapes. Inset is defined
by CSS Shapes Level 1 (http://dev.w3.org/csswg/css-shapes-1/#supported-basic-shapes).
Inset is going to be used by shape-outside, and eventually by shape-inside.

Review URL: https://codereview.chromium.org/107513002

git-svn-id: svn://svn.chromium.org/blink/trunk@165902 bbb929c8-8fbe-4397-9dbb-9b2b20218538