[iOS] Add support for the escape gesture

This CL adds a way to dismiss the IdentityChooser popup using the
escape gesture.

Bug: 872277
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ia0b64c28787ed651596e936aa806ed42b94e50db
Reviewed-on: https://chromium-review.googlesource.com/1172431Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582584}

ios/chrome/browser/ui/authentication/unified_consent/identity_chooser/identity_chooser_view_controller.mm

@@ -75,4 +75,11 @@ const CGFloat kFooterHeight = 17.;
   }
 }
 
+#pragma mark UIAccessibilityAction
+
+- (BOOL)accessibilityPerformEscape {
+  [self dismissViewControllerAnimated:YES completion:nil];
+  return YES;
+}
+
 @end