Oilpan: remove now-unused WebNode(Member<T>) constructor.

The (implicit) construction of a WebNode from a Member<T> (where T is
convertible to a Node) was introduced to allow for assignment of a
Member<Element> to a WebNode (as needed by http://crrev.com/280123002/)

With http://crrev.com/313813002 (r175522), an operator conversion from
Member<T> to RawPtr<T> was introduced, which makes the reason for having
the above WebNode() constructor fall away. Remove it here.

R=abarth@chromium.org
BUG=357163
NOTRY=true

Review URL: https://codereview.chromium.org/400473002

git-svn-id: svn://svn.chromium.org/blink/trunk@178296 bbb929c8-8fbe-4397-9dbb-9b2b20218538