Add SVGTests attrs to SVGSMILElement to support SVGDiscardElement

Before this CL, SVGAnimationElement derived SVGTests.
However, given that SVGDiscardElement, which also derives SVGSMILElement
need SVGTests support too, this CL changes SVGTests to be inherited in
SVGSMILElement.

Before:
class SVGSMILElement {}
class SVGAnimationElement : SVGSMILElement, SVGTests {}
class SVGDiscardElement : SVGSMILElement {}

After:
class SVGSMILElement : SVGTests {}
class SVGAnimationElement : SVGSMILElement {}
class SVGDiscardElement : SVGSMILElement {}

BUG=397991

Review URL: https://codereview.chromium.org/427613006

git-svn-id: svn://svn.chromium.org/blink/trunk@179107 bbb929c8-8fbe-4397-9dbb-9b2b20218538