Update device_second_factor_authentication device policy enum

As requested, add an 'Unset' value for the enum controlling the mode of
the integrated second factor authentication feature rather than using
the 'Disabled' state.

Bug: b/35545754
Change-Id: I3438eff45ba5740660cd313e0375d994a2a6d391
Reviewed-on: https://chromium-review.googlesource.com/548035
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Thiemo Nagel <tnagel@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#482239}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 57d9479f0ba28f1635d80c82c61d7d8582ec8fcd

chrome_device_policy.proto

diff --git a/chrome_device_policy.proto b/chrome_device_policy.proto
index eaffd32..e12ba92 100644
--- a/chrome_device_policy.proto
+++ b/chrome_device_policy.proto
@@ -799,15 +799,18 @@
 // functionality.
 message DeviceSecondFactorAuthenticationProto {
   enum U2fMode {
+    // Default value, unspecified.
+    UNSET = 0;
     // Feature disabled.
-    DISABLED = 0;
-    // U2F as defined by the FIDO Alliance specification.
-    U2F = 1;
+    DISABLED = 1;
+    // U2F as defined by the FIDO Alliance specification:
+    // https://fidoalliance.org/specs/fido-u2f-v1.1-id-20160915.zip
+    U2F = 2;
     // U2F plus extensions for individual attestation certificate.
-    U2F_EXTENDED = 2;
+    U2F_EXTENDED = 3;
   };
 
-  optional U2fMode mode = 1 [default = DISABLED];
+  optional U2fMode mode = 1;
 }
 
 message ChromeDeviceSettingsProto {