Reset scoped_ptr to make plugin crash on higher level.

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
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=273547

[gene1, 2014-05-28 23:40:56]

lgtm

[Vitaly Buka (NO REVIEWS), 2014-05-28 23:41:45]

The CQ bit was checked by vitalybuka@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-05-28 23:42:05]

The CQ bit was unchecked by vitalybuka@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-05-28 23:43:19]

The CQ bit was checked by vitalybuka@chromium.org

[commit-bot: I haz the power, 2014-05-28 23:44:46]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vitalybuka@chromium.org/300403002/1

[gene, 2014-05-29 01:57:42]

lgtm

[commit-bot: I haz the power, 2014-05-29 04:08:00]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  chromium_presubmit on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/chromium_presubmit/bu...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[Vitaly Buka (NO REVIEWS), 2014-05-29 05:23:58]

The CQ bit was unchecked by vitalybuka@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-05-29 05:25:17]

The CQ bit was checked by vitalybuka@chromium.org

[commit-bot: I haz the power, 2014-05-29 05:25:47]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vitalybuka@chromium.org/300403002/1

[commit-bot: I haz the power, 2014-05-29 19:38:05]

Change committed as 273547