SVG: Items inserted into list should be made a tear-off.

Before this patch, static SVGPropertyTearOff items inserted into a SVGListTearOff were not marked as a tear-off.
For such items, |svgAttributeChanged| was not dispatched to its context element, and view was not updated when its value changed.

This patch fixes the issue by converting a static tear-off inserted into a list to a dynamic tear-off.

Also this patch fixes an issue that a tearoff returned from replaceItem/removeItem was still attached to the list.

BUG=380546, 380176

Review URL: https://codereview.chromium.org/321403004

git-svn-id: svn://svn.chromium.org/blink/trunk@176284 bbb929c8-8fbe-4397-9dbb-9b2b20218538