[iOS][Alert] Add swipe gesture recognizer

Adds a swipe gesture recognizer that dismiss the keyboard when the alert
is swipped down.

Bug: 960417
Change-Id: I1179a158f9860754278ce73f1ee068ccbbaf2d8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602642
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659000}

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

@@ -95,6 +95,9 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
 // view.
 @property(nonatomic, strong) UITapGestureRecognizer* tapRecognizer;
 
+// Recognizer used to dismiss the keyboard swipping down the alert.
+@property(nonatomic, strong) UISwipeGestureRecognizer* swipeRecognizer;
+
 // This is the last focused text field, the gestures to dismiss the keyboard
 // will end up calling |resignFirstResponder| on this.
 @property(nonatomic, weak) UITextField* lastFocusedTextField;
@@ -128,6 +131,13 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
   self.contentView.translatesAutoresizingMaskIntoConstraints = NO;
   [self.view addSubview:self.contentView];
 
+  self.swipeRecognizer = [[UISwipeGestureRecognizer alloc]
+      initWithTarget:self
+              action:@selector(dismissKeyboard)];
+  self.swipeRecognizer.direction = UISwipeGestureRecognizerDirectionDown;
+  self.swipeRecognizer.enabled = NO;
+  [self.contentView addGestureRecognizer:self.swipeRecognizer];
+
   BOOL isAccessibilityContentSize =
       UIContentSizeCategoryIsAccessibilityCategory(
           [UIApplication sharedApplication].preferredContentSizeCategory);
@@ -429,11 +439,13 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
   self.additionalSafeAreaInsets =
       UIEdgeInsetsMake(0, 0, keyboardFrame.size.height, 0);
   self.tapRecognizer.enabled = YES;
+  self.swipeRecognizer.enabled = YES;
 }
 
 - (void)handleKeyboardWillHide:(NSNotification*)notification {
   self.additionalSafeAreaInsets = UIEdgeInsetsZero;
   self.tapRecognizer.enabled = NO;
+  self.swipeRecognizer.enabled = NO;
 }
 
 - (void)didSelectActionForButton:(UIButton*)button {