Add InvalidationStateTracker::Forget() to erase an entry from storage.

sync/tools/sync_client.cc

Index: sync/tools/sync_client.cc
diff --git a/sync/tools/sync_client.cc b/sync/tools/sync_client.cc
index 9047d482a4ccddabf1065b2e3453461204e9629a..5226fa9cf3a7a23f465d587aec2443913891a14e 100644
--- a/sync/tools/sync_client.cc
+++ b/sync/tools/sync_client.cc
@@ -83,6 +83,12 @@ class NullInvalidationStateTracker
             << ObjectIdToString(id) << " to " << max_invalidation_version;
   }
 
+  virtual void Forget(const ObjectIdSet& ids) OVERRIDE {
+    for (ObjectIdSet::const_iterator it = ids.begin(); it != ids.end(); ++it) {
+      VLOG(1) << "Forgetting invalidation state for " << ObjectIdToString(*it);
+    }
+  }
+
   virtual std::string GetInvalidationState() const OVERRIDE {
     return std::string();
   }