LibCore+LibWebView: Restore was_exit_requested to EventLoop

This method was removed in e015a43b51

However, it was not exactly *unused* as the commit message would say.
This method was the only thing that allowed spin_until to exit when
the event loop was cancelled. This happens normally when IPC connections
are closed, but also when the process is killed.

The logic to properly handle process exit from event loop spins needs to
actually notify the caller that their goal condition was not met though.
That will be handled in a later commit.

Libraries/LibCore/EventLoop.h

@@ -78,6 +78,8 @@ public:
 
     void quit(int);
 
+    bool was_exit_requested();
+
     // The registration functions act upon the current loop of the current thread.
     static intptr_t register_timer(EventReceiver&, int milliseconds, bool should_reload, TimerShouldFireWhenNotVisible);
     static void unregister_timer(intptr_t timer_id);