Use supplied reference box for layout using CSS Shapes

When the shape-inside or shape-outside properties have a <shape> <box> pair,
use the <box> to size and position the shape. When no box is supplied, use
the margin box for shape-outside and the content box for shape-inside. The
new behavior no longer relies on the box-sizing property, and changes the
default reference box for shape-outside, so some test corrections are also
included.

BUG=322165

Review URL: https://codereview.chromium.org/110573006

git-svn-id: svn://svn.chromium.org/blink/trunk@164811 bbb929c8-8fbe-4397-9dbb-9b2b20218538