[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..c8111a141111e22ac9d6a7c4827d78034c1353f7 100644
--- a/sync/protocol/sync.proto
+++ b/sync/protocol/sync.proto
@@ -730,7 +730,11 @@ message DataTypeContext {
 }
 
 message ClientToServerMessage {
+  // |share| field is only used on the server for logging and can sometimes
+  // contain empty string. It is still useful for logging username when it can't
+  // be derived from access token in case of auth error.
   required string share = 1;
+
   optional int32 protocol_version = 2 [default = 45];
   enum Contents {
     COMMIT = 1;