Drop application handler on run loop exit (#4149)

Calling the `Drop` impl of the user's `ApplicationHandler` is important on
iOS and Web, since they don't return from `EventLoop::run_app`.

And now that we reliably call `Drop`, the `ApplicationHandler::exited`
event/callback is unnecessary; using `Drop` composes much better (open files
etc. stored in the app state will be automatically flushed), and prevents
weirdness like attempting to create a new window while exiting.

src/platform_impl/web/event_loop/runner.rs

@@ -158,7 +158,6 @@ impl Runner {
             Event::Resumed => self.app.resumed(&self.event_loop),
             Event::CreateSurfaces => self.app.can_create_surfaces(&self.event_loop),
             Event::AboutToWait => self.app.about_to_wait(&self.event_loop),
-            Event::LoopExiting => self.app.exiting(&self.event_loop),
         }
     }
 }
@@ -639,7 +638,9 @@ impl Shared {
         self.apply_control_flow();
         // We don't call `handle_loop_destroyed` here because we don't need to
         // perform cleanup when the Web browser is going to destroy the page.
-        self.handle_event(Event::LoopExiting);
+        //
+        // We do want to run the application handler's `Drop` impl.
+        *self.0.runner.borrow_mut() = RunnerEnum::Destroyed;
     }
 
     // handle_event takes in events and either queues them or applies a callback
@@ -737,7 +738,6 @@ impl Shared {
     }
 
     fn handle_loop_destroyed(&self) {
-        self.handle_event(Event::LoopExiting);
         let all_canvases = std::mem::take(&mut *self.0.all_canvases.borrow_mut());
         *self.0.page_transition_event_handle.borrow_mut() = None;
         *self.0.on_mouse_move.borrow_mut() = None;
@@ -879,6 +879,5 @@ pub(crate) enum Event {
     CreateSurfaces,
     Resumed,
     AboutToWait,
-    LoopExiting,
     UserWakeUp,
 }