[ios] Updates InfobarBanner A11y actions for banners with no modal.

- Stops Adding the "Show Modal" A11y custom action for ConfirmInfobars
banners.

Bug: 1021597
Change-Id: I91ca187fbb7f70730af3cbfaf3c293140c6d06db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900138
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713058}

ios/chrome/browser/ui/infobars/banners/infobar_banner_view_controller.mm

@@ -522,14 +522,20 @@ - (NSArray*)accessibilityActions {
                 target:self
               selector:@selector(dismiss)];
 
-  UIAccessibilityCustomAction* modalAction =
-      [[UIAccessibilityCustomAction alloc]
-          initWithName:l10n_util::GetNSString(
-                           IDS_IOS_INFOBAR_BANNER_OPTIONS_HINT)
-                target:self
-              selector:@selector(triggerInfobarModal)];
+  NSMutableArray* accessibilityActions =
+      [@[ acceptAction, dismissAction ] mutableCopy];
+
+  if (self.presentsModal) {
+    UIAccessibilityCustomAction* modalAction =
+        [[UIAccessibilityCustomAction alloc]
+            initWithName:l10n_util::GetNSString(
+                             IDS_IOS_INFOBAR_BANNER_OPTIONS_HINT)
+                  target:self
+                selector:@selector(triggerInfobarModal)];
+    [accessibilityActions addObject:modalAction];
+  }
 
-  return @[ acceptAction, dismissAction, modalAction ];
+  return accessibilityActions;
 }
 
 // A11y Custom actions selectors need to return a BOOL.