[CSS Shapes] Refactor RectangleShape

Privatize and rename the FloatRoundedRect class defined in RectangleShape.h.

The new class is called RectangleShape::ShapeBounds. This change enables creating a proper FloatRoundedRect analog of the existing RoundedRect class; part of adding support for box shapes, per the latest CSS spec.

BUG=319946

Review URL: https://codereview.chromium.org/68493004

git-svn-id: svn://svn.chromium.org/blink/trunk@162240 bbb929c8-8fbe-4397-9dbb-9b2b20218538