Clear statement before closing db in cookie code.

Clear statement before closing db in cookie code.

sql::Statement maintains a weak ref to the associated sql::Connection,
meaning that if the database and statement are destructed in the wrong
order, a use-after-free can result.  sql::Statement::Clear() allows
resetting the statement to the default-constructed state.

BUG=111376
TEST=fewer crashes.


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=122430

[Scott Hess - ex-Googler, 2012-02-16 21:34:04]

I tested this by manually breaking the database format, converting some
DCHECK-type cases to logging, and using MallocScribble=1 when running Chrome. 
In that case, trunk crashed in the same place as described on the bug.  I made
just the smt.Close() change, and now it crashed dereferencing a NULL scoped_ptr.
 I made the load_success change and it no longer crashed, web sites could be
browsed, etc.  Obviously the user has no stored cookies, but their browser
should work.

Apologies for the long turn-around time, I was aiming for something that could
be justified to merged aggressively.

I also tried Assign(NULL), but sql::Statement code assumes that the ref is
non-NULL.  I also implemented a version which added an appropriately-scoped
Connection reference that the db_ ref could swap to, and it worked, but looked
ugly as sin.  Likewise, making smt a scoped_ptr seemed generally gross.

[Scott Hess - ex-Googler, 2012-02-16 21:52:14]

Adding Greg so he can track it WRT his change-which-will-need-a-merge.

[Scott Hess - ex-Googler, 2012-02-16 23:07:57]

Eric, Erik gave a broad OK to this approach at:
  https://chromiumcodereview.appspot.com/9418021/
I IM'ed him w/o response, so I'm guessing he's away from his desk.  Looking for
an OWNERS LGTM so that I can push the little button so that I don't have this
push back another day.  thx.

[erikwright (departed), 2012-02-17 00:22:26]

LGTM. Thanks Scott!

[commit-bot: I haz the power, 2012-02-17 00:28:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/shess@chromium.org/9418021/1

[commit-bot: I haz the power, 2012-02-17 02:15:10]

Change committed as 122430