Default unspecified value added to enum in proto

chrome/browser/chromeos/policy/proto/chrome_device_policy.proto

Index: chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
diff --git a/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto b/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
index 2330699f75581f3cf12bd73f3904791b10c27b75..cfdeca2b66678312982878cea8a454639f7e9850 100644
--- a/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
+++ b/chrome/browser/chromeos/policy/proto/chrome_device_policy.proto
@@ -783,10 +783,12 @@ message DeviceWallpaperImageProto {
 // the device used ecryptfs in the past.
 message DeviceEcryptfsMigrationStrategyProto {
   enum MigrationStrategy {
+    // Default value, unspecified.
+    MIGRATION_STRATEGY_UNSPECIFIED = 0;

[Andrew T Wilson (Slow), 2017/05/22 12:30:56]

I'm not sure that's what we want. What behavior do you expect if
MIGRATION_STRATEGY_UNSPECIFIED is provided? Is the implication that DISALLOW_ARC
should not be the default for unspecified values?

[bartfab (slow), 2017/05/22 13:23:47]

We (the client) do not want or need an UNSET value. What you are seeing here are
implementation details leaking from server to client. On the server, they want
to keep track of domains/OUs that explicitly set the policy vs. ones that never
set it. They need a sepecial UNSET value for that. Since server-side storage
uses the same proto as the client-server protocol (a long-known WTF in the
design), we have to add the value here if we want to help out the server guys.

Igor, let's call this simply UNSET and note in the comment that this triggers
version- and board-specific, undefined behavior and should thus never be sent by
the server.

[igorcov, 2017/05/22 17:20:32]

Changed to UNSET. Thanks for explanations.

[Thiemo Nagel, 2017/05/23 09:27:31]

Imho this is just enforcing proto best practices (which I think is fair, BTW). 
I don't think there's a specific need for this on the server side.

     // ARC is not allowed, no data migration needed.
-    DISALLOW_ARC = 0;
+    DISALLOW_ARC = 1;
     // The data migration is allowed, opening the possibility to use ARC.
-    ALLOW_MIGRATION = 1;
+    ALLOW_MIGRATION = 2;
   };
 
   optional MigrationStrategy migration_strategy = 1;