Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate.mm

Index: ios/web/web_state/web_state_delegate.mm
diff --git a/ios/web/web_state/web_state_delegate.mm b/ios/web/web_state/web_state_delegate.mm
index 7722a61bf451f8922c1b2eb048c7e32209bcb198..d08ee503787cdf3d37789f91e356ca5744df7e2a 100644
--- a/ios/web/web_state/web_state_delegate.mm
+++ b/ios/web/web_state/web_state_delegate.mm
@@ -24,6 +24,11 @@ bool WebStateDelegate::HandleContextMenu(WebState*, const ContextMenuParams&) {
   return false;
 }
 
+JavaScriptDialogPresenter* WebStateDelegate::GetJavaScriptDialogPresenter(
+    WebState*) {
+  return nullptr;
+}
+
 void WebStateDelegate::Attach(WebState* source) {
   DCHECK(attached_states_.find(source) == attached_states_.end());
   attached_states_.insert(source);