Add public API for handling Javascript alerts and prompt.

ios/web/web_state/web_state_delegate_stub.mm

Index: ios/web/web_state/web_state_delegate_stub.mm
diff --git a/ios/web/web_state/web_state_delegate_stub.mm b/ios/web/web_state/web_state_delegate_stub.mm
index 53d0de4bc5899f4347d47b50bcba5329eea70f28..000d8426ed95d3f30623270d246ccdafcf005a54 100644
--- a/ios/web/web_state/web_state_delegate_stub.mm
+++ b/ios/web/web_state/web_state_delegate_stub.mm
@@ -10,6 +10,8 @@
 @implementation CRWWebStateDelegateStub {
   // Backs up the property with the same name.
   std::unique_ptr<web::ContextMenuParams> _contextMenuParams;
+  // Backs up the property with the same name.
+  BOOL _javaScriptDialogPresenterRequested;
 }
 
 @synthesize webState = _webState;
@@ -27,8 +29,19 @@
   return YES;
 }
 
+- (web::JavaScriptDialogPresenter*)javaScriptDialogPresenterForWebState:
+    (web::WebState*)webState {
+  _webState = webState;
+  _javaScriptDialogPresenterRequested = YES;
+  return nil;
+}
+
 - (web::ContextMenuParams*)contextMenuParams {
   return _contextMenuParams.get();
 }
 
+- (BOOL)javaScriptDialogPresenterRequested {
+  return _javaScriptDialogPresenterRequested;
+}
+
 @end