Implement MathML DOM

See https://groups.google.com/forum/#!topic/mozilla.dev.platform/ssTytf-pT7k

Differential Revision: https://phabricator.services.mozilla.com/D45034

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1571487
gecko-commit: 30154d163acaa92552ed3c2d601fb0c658be44eb
gecko-integration-branch: autoland
gecko-reviewers: bzbarsky

Author: fred-wang

mathml/relations/html5-tree/tabindex-001.html

@@ -14,16 +14,20 @@
           const mrow = document.getElementById('mrow');
           assert_equals(mrow.tabIndex, -1, "no attribute");
           mrow.setAttribute("tabindex", "invalid");
+          assert_equals(mrow.getAttribute("tabindex"), "invalid");
           assert_equals(mrow.tabIndex, -1, "invalid");
           mrow.setAttribute("tabindex", "9999999999");
+          assert_equals(mrow.getAttribute("tabindex"), "9999999999");
           assert_equals(mrow.tabIndex, -1, "too large integer");
       }, "default and invalid values on mrow");
       test(() => {
           const mrowLink = document.getElementById('mrow-link');
           assert_equals(mrow.tabIndex, 0, "no attribute");
           mrow.setAttribute("tabindex", "invalid");
+          assert_equals(mrow.getAttribute("tabindex"), "invalid");
           assert_equals(mrow.tabIndex, 0, "invalid");
           mrow.setAttribute("tabindex", "9999999999");
+          assert_equals(mrow.getAttribute("tabindex"), "9999999999");
           assert_equals(mrow.tabIndex, 0, "too large integer");
       }, "default and invalid values on MathML link");
   });