Merge duplicate SVGGeometryElement+SVGPathElement entries

This was tested by checking for the 3 members on all 7 types of elements
which currently inherit from SVGGeometryElement:

http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=9076

```js
var tags = ['circle', 'ellipse', 'line', 'path', 'polygon', 'polyline', 'rect'];
var members = ['getPointAtLength', 'getTotalLength', 'pathLength'];
tags.forEach(function(localName) {
  var e = document.createElementNS('http://www.w3.org/2000/svg', localName);
  members.forEach(function(member) {
    w(localName + '.' + member + ': ' + typeof e[member]);
  });
});
```

In Chromium, getTotalLength and getPointAtLength were moved in M56,
and pathLength was moved in M57:
https://bugs.chromium.org/p/chromium/issues/detail?id=596043
https://storage.googleapis.com/chromium-find-releases-static/887.html#887bbf6c0edcabfa012099379c2d68a3dc4afd3f
https://storage.googleapis.com/chromium-find-releases-static/6fd.html#6fd1cbb3c5a636ee3830729e4b26ca1d5d78c8d6

In Gecko, the SVGGeometryElement was introduced in 53, but elements
other than <path> were made to inherit in 61:
https://bugzilla.mozilla.org/show_bug.cgi?id=1239100
https://bugzilla.mozilla.org/show_bug.cgi?id=1325320

In WebKit the change was made at trunk version 606.1.14:
https://trac.webkit.org/changeset/230829/webkit
https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/Configurations/Version.xcconfig?rev=230829

In Edge 13 and IE 11, only path.getPointAtLength and path.getTotalLength
were supported, there was no support for pathLength.

In Opera 12.16, all 3 members were supported on path and nowhere else.