Allow construction and assignment of one scoped_ptr from another if the types are convertible.

Allow construction and assignment of one scoped_ptr from another if the types are convertible.

This allows for depth subtyping during a move operation on a scoped_ptr. With the additional constructor and operator=, we maintain move semantics but allow a scoped_ptr<A> to be constructed from a scoped_ptr<B> if B can be converted to A.

=DEFICIENCY=
This conversion sequence will _not_ implicitly work when calling an API. Specifically, if you have
void Func(scoped_ptr<Parent> p);

  scoped_ptr<Child> c;
  Func(c.Pass()); // COMPILE ERROR

This is a limitation of C++03 move emulation.  The workaround is Func(scoped_ptr<Parent>(c.Pass());

BUG=109026
TEST=new unittests

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=117317

[Ami GONE FROM CHROMIUM, 2011-12-30 01:02:22]

FYI
Patched in the CL but ran into difficulties trying to take advantage of it in a
case where an API expected scoped_ptr<B> but a call-site was trying to pass
d2.Pass(), where 
d2 is an object of type D2 which derives from D which derives from B.

Adding an explicit scoped_ptr<B>(d2.Pass()) made it compile (as did having an
explicit variable of type scoped_ptr<B> initialized on d2.Pass() and Pass()'ing
that variable to the API).

[awong, 2012-01-11 00:55:51]

Hey Darin,

Would you be willing to review this?  Both Fred and Will are out. :(

-Albert

[darin (slow to review), 2012-01-11 01:17:54]

LGTM

[commit-bot: I haz the power, 2012-01-11 01:37:45]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ajwong@chromium.org/8968032/11001

[commit-bot: I haz the power, 2012-01-11 02:30:40]

Try job failure for 8968032-11001 (retry) on linux_rel for step "browser_tests"
(clobber build).
It's a second try, previously, step "compile" failed.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_rel&...