Bug 705154 - Hang monitor incorrectly killing the app on mac when the user is idle...

Bug 705154 - Hang monitor incorrectly killing the app on mac when the user is idle: unlike other platforms, the main loop on Mac doesn't call processNextEvent in a loop: instead it calls into [NSApplication run] and we check whether there are pending XPCOM events during processing. Suspend the hang monitor when we pause for incoming events by subclassing [NSApplication nextEventMatchingMask], r=smichaud

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extra : rebase_source : 753343ca11d3ce8b6801a5be8831621887fbe70b