Parse new <shape> <box> syntax for shape properties

The shape-inside and shape-outside properties now support basic shapes
with an optional box model keyword before or after them. This patch
adds support for parsing this new syntax.

BUG=322165

Review URL: https://codereview.chromium.org/105833009

git-svn-id: svn://svn.chromium.org/blink/trunk@164114 bbb929c8-8fbe-4397-9dbb-9b2b20218538