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

sync/notifier/fake_invalidation_state_tracker.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sync/notifier/fake_invalidation_state_tracker.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 
@@ skipping 17 matching lines @@
   }
   versions_[id] = max_version;
 }
 
 int64 FakeInvalidationStateTracker::GetMaxVersion(
     const invalidation::ObjectId& id) const {
   InvalidationVersionMap::const_iterator it = versions_.find(id);
   return (it == versions_.end()) ? kMinVersion : it->second;
 }
 
+void FakeInvalidationStateTracker::Forget(const invalidation::ObjectId& id) {
+  versions_.erase(id);
+}
+
 void FakeInvalidationStateTracker::SetInvalidationState(
     const std::string& state) {
   state_ = state;
 }
 
 std::string FakeInvalidationStateTracker::GetInvalidationState() const {
   return state_;
 }
 
 }  // namespace syncer