Call reset(nullptr) in scoped_ptr's destructor.

This behaviour is consistent with libc++.

The original goal of this change is to make Chrome crash more often
and closer to the source when there's a use-after-free bug that
accesses a destroyed scoped_ptr.

BUG=535321

Review URL: https://codereview.chromium.org/1356483002

Cr-Commit-Position: refs/heads/master@{#357297}
2 files changed