[CSS Shapes] Simplify the BoxShape implementation

Reduce BoxShape's footprint by about 2/3rds. Instead of caching the FloatRoundedRects which represent a BoxShape's shape-padding and shape-margin boundaries, compute them as needed.

No new tests, this is just an internal refactoring.

BUG=327493

Review URL: https://codereview.chromium.org/105063004

git-svn-id: svn://svn.chromium.org/blink/trunk@163745 bbb929c8-8fbe-4397-9dbb-9b2b20218538