SVGSMILElement should respect SVGTests results

SVGAnimationElement derives SVGTests, but it never checked for its results.
This patch modifies |SVGSMILElement::progress| so it will query |SVGElement::isValid()| before applying animation.

BUG=397991

Review URL: https://codereview.chromium.org/424753002

git-svn-id: svn://svn.chromium.org/blink/trunk@179122 bbb929c8-8fbe-4397-9dbb-9b2b20218538