[SVG] SVG*List.{initialize,reset} should detach its values.

SVGProperty value can be a member item of a list. When a list item value is appended to other list, the list item is removed from the original list, and then appended to the other list.

When an item was attached from previously cleared list, the item still contained old |ownerList| information which is invalid. This CL fixes the issue by clearing |ownerList| information from its values when list is cleared.

This CL moves |ownerList| from values' TearOffs to values itself to enable access from SVG*List impl.

This CL also encapsulates access to NewSVGPropertyListHelper<T>::m_values. After this change, the helper derived classes should also use public interface to access m_values. This is to ensure |ownerList| attach/detach operation is done correctly.

TBR=pdr@chromium.org
TEST=svg/dom/SVGLengthList-appendItemFromClearedList.html
BUG=308818

Review URL: https://codereview.chromium.org/144853002

git-svn-id: svn://svn.chromium.org/blink/trunk@165617 bbb929c8-8fbe-4397-9dbb-9b2b20218538