Always reset the animated property for CSS-type SVG animation

When resetting the animated property value for animations that is applied
as a CSS property always create a new property, or additive animations
will accumulate incorrectly to the underlying value.

BUG=349052

Review URL: https://codereview.chromium.org/183973023

git-svn-id: svn://svn.chromium.org/blink/trunk@168475 bbb929c8-8fbe-4397-9dbb-9b2b20218538