Update scoped_array<T>::reset logic to match scoped_ptr.

Update scoped_array<T>::reset logic to match scoped_ptr.

This logic was already added in scoped_ptr::reset() in
https://src.chromium.org/viewvc/chrome?view=rev&revision=185551.
this is a straightforward port of the logic to scoped_array to
prevent any accidental regressions during the transition.

BUG=162971, 176091
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=192590

[dcheng, 2013-04-05 16:27:20]

Mark, mind reviewing this patch so I can get this landed? In practice, I don't
think there will be any new regressions between now and when I finish the
scoped_array transition, but I'd feel better if it was enforced by the code as
well.

[Mark Mentovai, 2013-04-05 17:15:43]

LGTM

[dcheng, 2013-04-05 17:18:58]

Committed patchset #2 manually as r192590 (presubmit successful).