fix(SVG): don't use unique_ptr

[zbaylin]

This was causing segfaults.

[bryphe]

Looks good to me, thanks @zbaylin !

Do you think we can add in the ref/unref functionality here, too?

void sk_svgdom_ref(const sk_svgdom_t* svg) {
    SkSafeRef(AsSVGDOM(svg));
}

void sk_svgdom_unref(const sk_svgdom_t *svg) {
    SkSafeUnref(AsSVGDOM(svg));
}