[Sync] Add sync api support for context changes

sync/api/sync_change.cc

Index: sync/api/sync_change.cc
diff --git a/sync/api/sync_change.cc b/sync/api/sync_change.cc
index 8dedd9cb1768fbb0b3f99aebe8ae38e0cad773f7..ca33fbe7ac367ede9211726f6323dfa42539db3e 100644
--- a/sync/api/sync_change.cc
+++ b/sync/api/sync_change.cc
@@ -27,6 +27,12 @@ bool SyncChange::IsValid() const {
   if (change_type_ == ACTION_INVALID || !sync_data_.IsValid())
     return false;
 
+  // Context changes must not have valid specifics.
+  if (change_type_ == CONTEXT_UPDATE &&
+      GetModelTypeFromSpecifics(sync_data_.GetSpecifics()) != UNSPECIFIED) {
+    return false;
+  }
+
   // Data from the syncer must always have valid specifics.
   if (!sync_data_.IsLocal())
     return IsRealDataType(sync_data_.GetDataType());