Move RoundedRect from core/platform/graphics to platform/geometry

Move the RoundedRect class and tests from core/platform/graphics into
the the geometry directory in the new platform module.

BUG=297477
R=eseidel@chromium.org, leviw@chromium.org

Review URL: https://codereview.chromium.org/25683006

git-svn-id: svn://svn.chromium.org/blink/trunk@158824 bbb929c8-8fbe-4397-9dbb-9b2b20218538