[Mac] Keep download buttons alive while processing events.
[Mac] Keep download buttons alive while processing events.

Other places which spin a nested loop pin the containing object in
place in case something fires a window close.  There are crashes which
indicate that this is happening here.

BUG=92390
TEST=monitor crash server.


Review URL: http://codereview.chromium.org/7715019

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@98286 0039d316-1c4b-4281-b951-d872f2087c98

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