Add short circuit case for self assignment...

Add short circuit case for self assignment, and add virtual destructor

Pickle is a base class of Message, which has a virtual destructor, so
it is better to have Pickle have a virtual destructor as well.

Add more defensive code in the case of self assignment.
**IF** self assignment was used, and the target was read-only,
then the existing code would have a memory fault.
I don't think this is currently possible in use cases, but
the defensize code is much better. (per suggestion from CPU
on last CL).

BUG=17027
BUG=17088
r=cpu

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=22826

[cpu_(ooo_6.6-7.5), 2009-08-03 20:35:13]

LGTM

http://codereview.chromium.org/159800/diff/1/2
File base/pickle.cc (right):

http://codereview.chromium.org/159800/diff/1/2#newcode69
Line 69: return *this;
I would not mind if there is a NOTREACHED() since self assignment is always an
error.

[wtc, 2011-09-21 23:37:04]

http://codereview.chromium.org/159800/diff/2001/base/pickle.h
File base/pickle.h (right):

http://codereview.chromium.org/159800/diff/2001/base/pickle.h#newcode34
base/pickle.h:34: virtual ~Pickle();
jar: I was wondering why the Pickle class has a virtual
destructor, and I tracked it down to this CL.

The Pickle class does not have any virtual method, so it
is strange for it to have a virtual destructor.

I am not convinced that the reason you gave is correct.
In any case, I suggest you add a comment to the source code
to explain why.  This will save the next person a few minutes
to answer this question.  Thanks.