Enable Null Syncable ID which is different than Root ID.

sync/syncable/syncable_id.h

 // Copyright 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.
 
 #ifndef SYNC_SYNCABLE_SYNCABLE_ID_H_
 #define SYNC_SYNCABLE_SYNCABLE_ID_H_
 
 #include <iosfwd>
 #include <limits>
 #include <sstream>
@@ skipping 25 matching lines @@
 // 2. Flag specifing whether server know about this object.
 // 3. Flag for root.
 //
 // We originally wrapped an integer for this information, but now we use a
 // string. It will have one of three forms:
 // 1. c<client only opaque id> for client items that have not been committed.
 // 2. r for the root item.
 // 3. s<server provided opaque id> for items that the server knows about.
 class SYNC_EXPORT Id {
  public:
-  // This constructor will be handy even when we move away from int64s, just
-  // for unit tests.
-  inline Id() : s_("r") { }
+  inline Id() : s_("") {}

[pavely, 2014/12/22 21:38:09]

Did you write s_("") to make it more explicit and readable? s_() will have the
same behavior.

[stanisc, 2014/12/22 21:56:01]

You are right. s_ would auto-initialize to an empty string. I think I ended up
having it this way because I simply edited the old initialization with "r",
replacing "r" with an empty string. I'll fix this.

[stanisc, 2014/12/22 22:44:20]

Done.

   inline Id(const Id& that) {
     Copy(that);
   }
   inline Id& operator = (const Id& that) {
     Copy(that);
     return *this;
   }
   inline void Copy(const Id& that) {
     this->s_ = that.s_;
   }
   inline bool IsRoot() const {
     return "r" == s_;
   }
   inline bool ServerKnows() const {
     return s_[0] == 's' || s_ == "r";
   }
 
-  // TODO(sync): We could use null here, but to ease conversion we use "r".
-  // fix this, this is madness :)
-  inline bool IsNull() const {
-    return IsRoot();
-  }
-  inline void Clear() {
-    s_ = "r";
-  }
+  inline bool IsNull() const { return s_.size() == 0; }

[pavely, 2014/12/22 21:38:09]

nit: 
replace "s_.size() == 0" with "s_.empty()"
On next line replace s_ = "" with s_.clear()

[stanisc, 2014/12/22 22:44:20]

Done.

+  inline void Clear() { s_ = ""; }
   inline int compare(const Id& that) const {
     return s_.compare(that.s_);
   }
   inline bool operator == (const Id& that) const {
     return s_ == that.s_;
   }
   inline bool operator != (const Id& that) const {
     return s_ != that.s_;
   }
   inline bool operator < (const Id& that) const {
@@ skipping 21 matching lines @@
   static Id CreateFromServerId(const std::string& server_id);
   // This should only be used if you get back a reference to a local
   // id from the server. Returns a client only opaque id.
   static Id CreateFromClientString(const std::string& local_id);
 
   // This method returns an ID that will compare less than any valid ID.
   // The returned ID is not a valid ID itself.  This is useful for
   // computing lower bounds on std::sets that are ordered by operator<.
   static Id GetLeastIdForLexicographicComparison();
 
+  // Gets root ID.
+  static Id GetRoot();
+
  private:
   friend scoped_ptr<EntryKernel> UnpackEntry(sql::Statement* statement);
   friend void BindFields(const EntryKernel& entry,
                          sql::Statement* statement);
   SYNC_EXPORT_PRIVATE friend std::ostream& operator<<(std::ostream& out,
                                                       const Id& id);
   friend class MockConnectionManager;
   friend class SyncableIdTest;
 
   std::string s_;
 };
 
-SYNC_EXPORT_PRIVATE Id GetNullId();
-
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_SYNCABLE_ID_H_