Only use SVG Elements for SVG animation events

This patch fixes a regression from [1] where we failed to check
that the animation target was an SVG element. This patch ensures
event bases are SVGElements in SVGSMILElement::eventBaseFor.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=164697

BUG=333431

Review URL: https://codereview.chromium.org/133993008

git-svn-id: svn://svn.chromium.org/blink/trunk@164921 bbb929c8-8fbe-4397-9dbb-9b2b20218538