scoped_refptr: support opaque pointed-to classes via extern template

scoped_refptr: support opaque pointed-to classes via extern template

The C++ spec says the compiler may instantiate inline template
function definitions (e.g., member functions defined inline within a
templated class, even without an explicit "inline" keyword) even if
there's an "extern template class Foo<Bar>;" declaration.

In the case of scoped_refptr, if we merely have a forward declaration
for a class like "class Opaque;", then "extern template class
scoped_refptr<Opaque>;" will still cause compiler errors because of
scoped_refptr's inline member functions that try to call AddRef() and
Release().

To solve this, this CL introduces static helper methods that are
defined out-of-line to provide access to the AddRef() and Release()
methods.

BUG=414363

Review URL: https://codereview.chromium.org/623633002

Cr-Commit-Position: refs/heads/master@{#298991}