Add an action for server to send down rollback command.

sync/protocol/sync_enums.proto

Index: sync/protocol/sync_enums.proto
diff --git a/sync/protocol/sync_enums.proto b/sync/protocol/sync_enums.proto
index 9305c677384a481f73009390f5936eee3f9c52ad..60c2e55899243de45f6baf041754952d83b175df 100644
--- a/sync/protocol/sync_enums.proto
+++ b/sync/protocol/sync_enums.proto
@@ -113,7 +113,9 @@ message SyncEnums {
     STOP_AND_RESTART_SYNC       = 3; // Stop sync and set up sync again.
     DISABLE_SYNC_ON_CLIENT      = 4; // Wipe the client of all sync data and
                                      // stop syncing.
-    UNKNOWN_ACTION              = 5; // This is the default.
+    DISABLE_SYNC_AND_ROLLBACK   = 5; // Wipe out sync data, stop syncing and
+                                     // roll back local data to pre-sync state.
+    UNKNOWN_ACTION              = 6; // This is the default.

[Nicolas Zea, 2014/04/15 23:08:31]

it's a bit concerning that we're modifying the value. I suppose that's okay if
we don't persist it anywhere.

[maniscalco, 2014/04/15 23:16:26]

I was wondering the same thing.  Is Action ever sent over the wire?

If old code (pre this CL) tries to decode a new Action field (post this CL),
won't it interpret the value as UPGRADE_CLIENT?  Check out this section, it says
the first value of the enum is the default:
https://developers.google.com/protocol-buffers/docs/proto#optional

[haitaol1, 2014/04/15 23:44:03]

I think it's by design that UNKNOWN_ACTION is put at the end so that new enums
won't break legacy client because they will be interpreted as unknown. See
ConvertClientActionPBToLocalClientAction()

On 2014/04/15 23:08:31, Nicolas Zea wrote:

[maniscalco, 2014/04/16 00:09:20]

To make things "obviously safe" I suggest leaving UNKNOWN_ACTION as 5, moving
UNKOWN_ACTION to the top of the enum (based on the doc I referenced above), and
adding the new DISABLE_SYNC_AND_ROLLBACK as 6.  This way, it's clear that
UNKNOWN_ACTION won't be misinterpreted as a DISABLE_SYNC_AND_ROLLBACK and it's
clear the default for an enum Action is UNKNOWN_ACTION.

On 2014/04/15 23:44:03, haitaol1 wrote:

   }
 
   enum DeviceType {