Oilpan: add Member<T>(RawPtr<U>) constructor.

r170929 introduced code that constructs a std::pair with a transition
type of OwnPtrWillBeMember<T> as the second component, from a value
having the transition type OwnPtrWillBeRawPtr<T>. With Oilpan enabled,
this requires an implicit conversion from RawPtr<T> to Member<T>,
which isn't supported. Introduce it here by adding a Member<T>
constructor from a given RawPtr<>.

R=
BUG=

Review URL: https://codereview.chromium.org/225303010

git-svn-id: svn://svn.chromium.org/blink/trunk@170942 bbb929c8-8fbe-4397-9dbb-9b2b20218538