Revert 273547 "Reset scoped_ptr to make plugin crash before ente..."

Revert 273547 "Reset scoped_ptr to make plugin crash before ente..."

> Reset scoped_ptr to make plugin crash before entering engine_->HandleDocumentLoad, if called for deleted "Instance".
> Suspecting method call for deleted instance.
>
> Example to explain why reset() makes difference:
>
> scoped_ptr<int>* pt = NULL;
> {
>   scoped_ptr<int> t(new int);
>   pt = &t;
> }
> DCHECK(!(*pt));  // Fail
> {
>   scoped_ptr<int> t(new int);
>   pt = &t;
>   t.reset();
> }
> DCHECK(!(*pt)); // OK
>
> BUG=372095,372548
>
> Review URL: https://codereview.chromium.org/300403002

TBR=vitalybuka@chromium.org

Review URL: https://codereview.chromium.org/318743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274854 0039d316-1c4b-4281-b951-d872f2087c98