[iOS][Getaway] Fall back to Passcode string correctly.

On a device with a biometric sensor which is disabled, LAContext's
biometryType will be set to the type of the sensor, even if it's not
set up. Check the error instead to see when a fallback to Passcode is
required.

Fixed: 1161175
Change-Id: I3db27208e46cd6aa11cf802e90086c1c040dc330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601356
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#840115}

ios/chrome/browser/ui/incognito_reauth/incognito_reauth_util.mm

@@ -15,8 +15,14 @@
 NSString* biometricAuthenticationTypeString() {
   LAContext* ctx = [[LAContext alloc] init];
   // Call canEvaluatePolicy:error: once to populate biometrics type
+  NSError* error;
   [ctx canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
-                   error:nil];
+                   error:&error];
+  if (error.code == LAErrorBiometryNotAvailable ||
+      error.code == LAErrorBiometryNotEnrolled) {
+    return l10n_util::GetNSString(IDS_IOS_INCOGNITO_REAUTH_PASSCODE);
+  }
+
   switch (ctx.biometryType) {
     case LABiometryTypeFaceID:
       return @"Face ID";