Revert of Add nullptr support to scoped_ptr.

Revert of Add nullptr support to scoped_ptr. (patchset #7 id:180001 of https://codereview.chromium.org/599313003/)

Reason for revert:
This CL broke several scoped_ptr tests.

Original issue's description:
> Add nullptr support to scoped_ptr.
>
> This adds support to use nullptr to construct, assign, or return a
> scoped_ptr<T> and scoped_ptr<T[]>. Support for this requires the use
> of a move-only constructor.
>
> The changes are:
>
> - Add a constructor that takes decltype(nullptr) as a parameter. This
> allows behaviour such as scoped_ptr<T>(nullptr), but also allows a
> function with return type scoped_ptr<T> to "return nullptr;" instead
> of "return scoped_ptr<T>();".
>
> - Add an operator=(decltype(nullptr)) that resets the scoped_ptr to
> empty and deletes anything it held.
>
> - Add/Modify a constructor to take a scoped_ptr<U,E>&& parameter for
> constructing a scoped_ptr from another using move-only semantics. This
> piece is critical for allowing the function returning nullptr to be
> assigned to some other scoped_ptr at the callsite. In particular, take
> the following code:
>   scoped_ptr<T> Function() { return nullptr; }
>   scoped_ptr<T> var = Function();
> In this case the constructor which takes a nullptr allows Function() to
> be written, but not to be used. The move-only constructor allows the
> assignment from Function() to var. See "C++11 feature proposal:
> Move-only constructors" on chromium-dev for more explanation why.
>
> The scoped_ptr<T> class already had a constructor which took
> scoped_ptr<U,E> as an argument, so this was changed to be
> scoped_ptr<U,E>&& instead. The scoped_ptr<T[]> class had no such
> constructor, so a scoped_ptr&& constructor was added. These match
> the constructors found on the unique_ptr class.
>
> - Remove the RValue type and the contructor that constructs a
> scoped_ptr from an RValue. Change Pass() to return a scoped_ptr&&
> instead of a scoped_ptr::RValue, to avoid the type conversion and
> remove some complexity. This is done with a new emulation macro that
> still provides Pass() and makes the type go down the MoveOnlyType
> path in base::Callback code.
>
> This adds base_unittests to demonstrate and use these changes.
>
> The use of Pass() remains unchanged until std::move() is written
> or allowed. At that time std::move() could be used instead of Pass.
>
> R=brettw@chromium.org, jamesr@chromium.org
>
> Committed: https://crrev.com/2299e91d3508f8d5d18ef990cf6024ea4371250a
> Cr-Commit-Position: refs/heads/master@{#297072}

TBR=brettw@chromium.org,jamesr@chromium.org,thakis@chromium.org,danakj@chromium.org
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/3e0ecf447b35e5bc984d9f00e192de335d5657a1
Cr-Commit-Position: refs/heads/master@{#297107}

[dcheng, 2014-09-27 07:34:58]

Created Revert of Add nullptr support to scoped_ptr.

[commit-bot: I haz the power, 2014-09-27 07:35:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/603353005/1

[commit-bot: I haz the power, 2014-09-27 07:35:26]

Committed patchset #1 (id:1) as 5eaf7f390158c105fca6eb1e5a53bd44696b25fb

[commit-bot: I haz the power, 2014-09-27 07:36:11]

Patchset 1 (id:??) landed as
https://crrev.com/3e0ecf447b35e5bc984d9f00e192de335d5657a1
Cr-Commit-Position: refs/heads/master@{#297107}