Add CRWNativeContent willBeDismissed.

ios/web/web_state/ui/crw_web_controller_container_view.mm

Index: ios/web/web_state/ui/crw_web_controller_container_view.mm
diff --git a/ios/web/web_state/ui/crw_web_controller_container_view.mm b/ios/web/web_state/ui/crw_web_controller_container_view.mm
index 25e31bdead7566ca9822665fe69d06cb203c3764..87e1363cd2070adcec77179444157a5e65e01167 100644
--- a/ios/web/web_state/ui/crw_web_controller_container_view.mm
+++ b/ios/web/web_state/ui/crw_web_controller_container_view.mm
@@ -172,6 +172,9 @@
 - (void)setNativeController:(id<CRWNativeContent>)nativeController {
   if (![_nativeController isEqual:nativeController]) {
     base::WeakNSProtocol<id> oldController(_nativeController);
+    if ([oldController respondsToSelector:@selector(willBeDismissed)]) {
+      [oldController willBeDismissed];
+    }
     [[oldController view] removeFromSuperview];
     _nativeController.reset([nativeController retain]);
     // TODO(crbug.com/503297): Re-enable this DCHECK once native controller