Add missing test to WeakPtr

Add missing test to WeakPtr

I discovered we have a weak pointer now and in my quest to understand how it works
I spotted case that was not tested:

1) ctor weakptr a
2) ctor factory f
3) ctor weakptr b
4) dtor weakptr b
5) dtor factory f
6) test a

Mostly because I thought I saw a mistake but alas it works as-is but still
the test is missing.

BUG=none
TEST= this is a test  
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=86526

[willchan no longer on Chromium, 2011-05-24 22:29:24]

LGTM

http://codereview.chromium.org/7065030/diff/1/base/memory/weak_ptr_unittest.cc
File base/memory/weak_ptr_unittest.cc (right):

http://codereview.chromium.org/7065030/diff/1/base/memory/weak_ptr_unittest.c...
base/memory/weak_ptr_unittest.cc:90: WeakPtr<int> b = factory.GetWeakPtr();
Maybe you should also add EXPECTs for b? Up to you.