Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_impl.h

Index: ios/web/web_state/web_state_impl.h
diff --git a/ios/web/web_state/web_state_impl.h b/ios/web/web_state/web_state_impl.h
index 776416c8e6baf81801535d5ca304b9a8be403ef4..521560bea74fb9775a1012997e1ffad30cd88318 100644
--- a/ios/web/web_state/web_state_impl.h
+++ b/ios/web/web_state/web_state_impl.h
@@ -20,6 +20,8 @@
 #include "ios/web/navigation/navigation_manager_delegate.h"
 #include "ios/web/navigation/navigation_manager_impl.h"
 #include "ios/web/net/request_tracker_impl.h"
+#import "ios/web/public/java_script_dialog_callback.h"
+#include "ios/web/public/java_script_dialog_type.h"
 #include "ios/web/public/web_state/web_state.h"
 #include "url/gurl.h"
 
@@ -267,6 +269,16 @@ class WebStateImpl : public WebState, public NavigationManagerDelegate {
   // Notifies the delegate that a context menu needs handling.
   bool HandleContextMenu(const ContextMenuParams& params);
 
+  // Notifies the delegate that a JavaScript dialog needs to be presented.
+  void RunJavaScriptDialog(const GURL& origin_url,
+                           JavaScriptDialogType java_script_dialog_type,
+                           NSString* message_text,
+                           NSString* default_prompt_text,
+                           const DialogClosedCallback& callback);
+
+  // Cancels all dialogs associated with this web_state.
+  void CancelActiveAndPendingDialogs();
+
   // NavigationManagerDelegate:
   void NavigateToPendingEntry() override;
   void LoadURLWithParams(const NavigationManager::WebLoadParams&) override;