Adds the ability for ObserverList to not notify observers added during...

Adds the ability for ObserverList to not notify observers added during
notification. I need this for bookmarks. If a new observer is added
while the bookmark model is in the process of sending out notification
the newly added observer gets confused.

BUG=674
TEST=none


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=4267

[darin (slow to review), 2008-10-30 18:15:13]

I don't think this solution works.  What if observers are removed as well as
added during notification?

[darin (slow to review), 2008-10-30 19:55:45]

> While sending out notifications a remove only nulls out the value, it doesn't
shrink the list. So, if you remove/add everything works fine (or if you remove
only).

Ah, right.  Sorry for overlooking that.  So, this patch is OK, but I wonder. 
Why make this a property of the ObserverList?  Why not just make it be a
property of the Iterator?  It seems like you could write a macro named
FOR_EACH_EXISTING_OBSERVER that would cause iteration over just the existing set
of observers.  That wouldn't require any new member variables on the
ObserverList.

[darin (slow to review), 2008-10-30 20:12:50]

LGTM

http://codereview.chromium.org/8919/diff/1/2
File base/observer_list.h (right):

http://codereview.chromium.org/8919/diff/1/2#newcode71
Line 71: };
nit: add a newline below here.