Implement parsing of the new ellipse shape syntax.

    This closely follows the patch for the new circle syntax,  
    with some refactoring of functionality shared by both.

    Updated existing parsing tests to cover this.

BUG=322165

Review URL: https://codereview.chromium.org/103413006

git-svn-id: svn://svn.chromium.org/blink/trunk@164465 bbb929c8-8fbe-4397-9dbb-9b2b20218538