Add public API for handling Javascript alerts and prompt.

ios/web/public/web_state/web_state_delegate.h

Index: ios/web/public/web_state/web_state_delegate.h
diff --git a/ios/web/public/web_state/web_state_delegate.h b/ios/web/public/web_state/web_state_delegate.h
index bebe64ea8f2772c5c0b20606d9f05505decda7f5..dc492a8586c7aca32a7df51160fcd63ad81f8925 100644
--- a/ios/web/public/web_state/web_state_delegate.h
+++ b/ios/web/public/web_state/web_state_delegate.h
@@ -16,6 +16,7 @@ class SSLInfo;
 namespace web {
 
 struct ContextMenuParams;
+class JavaScriptDialogPresenter;
 struct SSLStatus;
 class WebState;
 
@@ -36,6 +37,12 @@ class WebStateDelegate {
   virtual bool HandleContextMenu(WebState* source,
                                  const ContextMenuParams& params);
 
+  // Returns a pointer to a service to manage dialogs. May return nullptr in
+  // which case dialogs aren't shown.
+  // TODO(crbug.com/622084): Find better place for this method.
+  virtual JavaScriptDialogPresenter* GetJavaScriptDialogPresenter(
+      WebState* source);
+
  protected:
   virtual ~WebStateDelegate();