[iOS][Alert] Jumps to next field or hides keyboard

When the return key in the keyboard is pressed, the following text field
will become first responder. If there is none, the current text field
resigns first responder to hide the keyboard.

Bug: 960420, 951300
Change-Id: I6497bc9e1930b06054ab2946665e4836de94d428
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602497
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658512}

ios/chrome/browser/ui/alert_view_controller/alert_view_controller.mm

@@ -66,7 +66,7 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
 
 }  // namespace
 
-@interface AlertViewController ()
+@interface AlertViewController () <UITextFieldDelegate>
 
 // The actions for to this alert. |copy| for safety against mutable objects.
 @property(nonatomic, copy) NSArray<AlertAction*>* actions;
@@ -286,6 +286,7 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
       textField.accessibilityIdentifier =
           textFieldConfiguration.accessibilityIdentifier;
       textField.translatesAutoresizingMaskIntoConstraints = NO;
+      textField.delegate = self;
       [fieldStack addArrangedSubview:textField];
       ChromeDirectionalEdgeInsets fieldInsets = ChromeDirectionalEdgeInsetsMake(
           0.0, kTextfieldInset, 0.0, kTextfieldInset);
@@ -377,6 +378,18 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
   return results;
 }
 
+#pragma mark - UITextFieldDelegate
+
+- (BOOL)textFieldShouldReturn:(UITextField*)textField {
+  NSUInteger index = [self.textFields indexOfObject:textField];
+  if (index + 1 < self.textFields.count) {
+    [self.textFields[index + 1] becomeFirstResponder];
+  } else {
+    [textField resignFirstResponder];
+  }
+  return NO;
+}
+
 #pragma mark - Private
 
 - (void)handleKeyboardWillShow:(NSNotification*)notification {