[Sync] USS: Ignore encryption changes during conflict resolution 1/2

sync/protocol/entity_metadata.proto

Index: sync/protocol/entity_metadata.proto
diff --git a/sync/protocol/entity_metadata.proto b/sync/protocol/entity_metadata.proto
index 4b02cf61af0b7d543c4ac50d63b48178d54e0cc8..18e192d2eb15f94ce1dc4c83928cd51ce1a87ebc 100644
--- a/sync/protocol/entity_metadata.proto
+++ b/sync/protocol/entity_metadata.proto
@@ -52,4 +52,10 @@ message EntityMetadata {
   // entity's specifics value has changed without having to keep specifics in
   // memory.
   optional string specifics_hash = 9;
+
+  // A hash of the last specifics known by both the client and server. Used to
+  // detect when local commits and remote updates are just for encryption. This
+  // value will be the empty string only in the following cases: the entity is
+  // in sync with the server, has never been synced, or is deleted.
+  optional string base_specifics_hash = 10;
 }