[Sync] Add comment that ClientToServerMessage.share can be empty.

sync/protocol/sync.proto

Index: sync/protocol/sync.proto
diff --git a/sync/protocol/sync.proto b/sync/protocol/sync.proto
index f3cbbe58d58f0b11e85ed80ebe8ef5228d666328..f929f730b974d1c7b6dbe7bd5d30178373daa692 100644
--- a/sync/protocol/sync.proto
+++ b/sync/protocol/sync.proto
@@ -730,7 +730,10 @@ message DataTypeContext {
 }
 
 message ClientToServerMessage {
+  // |share| field is only used on the server for logging and can sometimes
+  // contain empty string. It should be deprecated.

[Nicolas Zea, 2015/12/30 20:16:23]

Could you mention also what's preventing us from deprecating this now?

[pavely, 2016/01/04 19:05:17]

I've updated comment. We shouldn't actually deprecate it yet. This field allows
to log username when access token is invalid.

   required string share = 1;
+
   optional int32 protocol_version = 2 [default = 45];
   enum Contents {
     COMMIT = 1;