Issue #2737 - Part 1: Base implementation of SVGGeometryElement.

Mostly mechanical changes to generalize path geometry for all SVG draw
elements. No user-exposed changes.

dom/webidl/SVGGeometryElement.webidl

@@ -0,0 +1,20 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * The origin of this IDL file is
+ * http://www.w3.org/TR/SVG2/
+ *
+ * Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C
+ * liability, trademark and document use rules apply.
+ */
+
+interface SVGGeometryElement : SVGGraphicsElement {
+  [SameObject]
+  readonly attribute SVGAnimatedNumber pathLength;
+
+  float getTotalLength();
+  [NewObject, Throws]
+  SVGPoint getPointAtLength(float distance);
+};