Update SVG interfaces to stop inheriting from SVGURIReference

Update SVG interfaces to stop inheriting from SVGURIReference

According to the latest specification, SVGAElement should implement
SVGURIReference, not inherit it.

This patch also updates other interfaces to use implements statements
instead of inheritance. While this is no longer part of the specification,
this was done to maintain backward compatibility.

This is one step towards getting rid of multiple inheritence in SVG.
There is no webexposed behavior change.

Specification: http://www.w3.org/TR/SVG2/linking.html#InterfaceSVGAElement
BUG=252181
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=152806

[haraken, 2013-06-20 12:38:46]

LGTM, but stephen might want to take a look.

[arv (Not doing code reviews), 2013-06-20 13:49:42]

LGTM

[Stephen Chennney, 2013-06-20 17:08:59]

lgtm

[commit-bot: I haz the power, 2013-06-20 17:09:13]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ch.dumez@sisa.samsung.com/17365004/1

[commit-bot: I haz the power, 2013-06-20 18:44:01]

Change committed as 152806