[SVG2] Add getters and setters to SVG*List interfaces.

Implement the 'length' attribute and make 'numberOfItems' an alias
of that.

Spec:
https://svgwg.org/svg2-draft/types.html#InterfaceSVGStringList
https://svgwg.org/svg2-draft/types.html#InterfaceSVGNumberList
https://svgwg.org/svg2-draft/types.html#InterfaceSVGLengthList
https://svgwg.org/svg2-draft/coords.html#InterfaceSVGTransformList
https://svgwg.org/svg2-draft/coords.html#InterfaceSVGPointList

BUG=336748

Review URL: https://codereview.chromium.org/195313003

git-svn-id: svn://svn.chromium.org/blink/trunk@169332 bbb929c8-8fbe-4397-9dbb-9b2b20218538