Make bubble more tolerant of notification order when quitting.
Make bubble more tolerant of notification order when quitting.

BUG=http://crbug.com/37807

TEST=\
Run a debug build of Chrome like this:  
  NSZombieEnabled=YES xcodebuild/Debug/Chromium.app/Contents/MacOS/Chromium 
Open a bubble then Cmd-Q.
Make sure you don't see a line like this:
  2010-03-10 19:24:20.381 Chromium[73024:903] *** -[ChromeBrowserWindow removeChildWindow:]: message sent to deallocated instance 0x18f8b2a0
Repeat 20 times.


Review URL: http://codereview.chromium.org/839003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@41342 0039d316-1c4b-4281-b951-d872f2087c98

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