Revert of cc: Remove use of PassAs() and constructor-casting with scoped_ptr. (patchs...

Revert of cc: Remove use of PassAs() and constructor-casting with scoped_ptr. (patchset #6 id:120001 of https://codereview.chromium.org/609663003/)

Reason for revert:
scoped_ptr nullptr support needs to be reverted

Original issue's description:
> cc: Remove use of PassAs() and constructor-casting with scoped_ptr.
>
> Say you have class A and subclass B.
>
> Previously it was required to PassAs() a scoped_ptr<B> into a
> scoped_ptr<A>. This is no longer needed, so just use Pass(). For newly
> created scoped_ptrs, you can just use make_scoped_ptr always now.
>
> And when you want to return or assign an empty scoped_ptr(), you can
> now use nullptr directly.
>
> Also adds PRESUBMIT checks for:
> - return scoped<T>(foo). This should be return make_scoped_ptr(foo).
> - bar = scoped<T>(foo). This should be return bar = make_scoped_ptr(foo).
> - return scoped<T>(). This should be return nullptr.
> - bar = scoped<T>(). This should be return bar = nullptr.
>
> This also replaces p.reset() with p = nullptr; But it does not add a
> PRESUBMIT check for that because there are things other than scoped_ptr
> with a reset() function.
>
> R=enne@chromium.org
>
> Committed: https://crrev.com/7bb3dbede19d87f0338797756ffd738adc6bca08
> Cr-Commit-Position: refs/heads/master@{#297096}

TBR=enne@chromium.org,jamesr@chromium.org,vmpstr@chromium.org,danakj@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/608503005

Cr-Commit-Position: refs/heads/master@{#297106}