[Mac] Ignore rapp events when the browser is shutting down.
[Mac] Ignore rapp events when the browser is shutting down.

BUG=42430
TEST=Suspected test case is to click on the dock icon immediately after quitting, but before the quit has finished.  This is very timing-dependent, and I have never actually been able to reproduce the crash.
Review URL: http://codereview.chromium.org/1796001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@45609 0039d316-1c4b-4281-b951-d872f2087c98

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