[CSS Shapes] Add FloatRoundedRect unit tests

Added a unit test and corrected a problem in FloatRoundedRect::xInterceptsAtY() (which is also part of the TBD patch for https://code.google.com/p/chromium/issues/detail?id=328180) - For the sake of computing X intercepts the bottom and right rectangle edges are considered part of the rectangle.

BUG=327892

Review URL: https://codereview.chromium.org/109793003

git-svn-id: svn://svn.chromium.org/blink/trunk@163883 bbb929c8-8fbe-4397-9dbb-9b2b20218538