sync: migrate invalidation state from syncable::Directory to InvalidationStorage

sync/notifier/invalidation_state_tracker.h

Index: sync/notifier/invalidation_state_tracker.h
diff --git a/sync/notifier/invalidation_state_tracker.h b/sync/notifier/invalidation_state_tracker.h
index 12568e6fbe02953f3f000c94741010e474a60708..bfa27c915d2fa09b462eba33125cdbb0a790bf0b 100644
--- a/sync/notifier/invalidation_state_tracker.h
+++ b/sync/notifier/invalidation_state_tracker.h
@@ -31,6 +31,7 @@ class InvalidationStateTracker {
   // Used by InvalidationClient for persistence. |state| is opaque data we can
   // present back to the client (e.g. after a restart) for it to bootstrap
   // itself.
+  // |state| is plain old data (not valid UTF8, embedded nulls, etc).
   virtual void SetInvalidationState(const std::string& state) = 0;
   virtual std::string GetInvalidationState() const = 0;