[CSS Shapes] Add support for the computing the included intervals for a BoxShape

Setting shape-inside to content-box now works. 

Corrected a mistake in BoxShape::getExcludedIntervals() that was exposed by the BoxShapeTest updates. 

BUG=329918

Review URL: https://codereview.chromium.org/124353002

git-svn-id: svn://svn.chromium.org/blink/trunk@164522 bbb929c8-8fbe-4397-9dbb-9b2b20218538