Enable Null Syncable ID which is different than Root ID.

sync/syncable/syncable_id.h

Index: sync/syncable/syncable_id.h
diff --git a/sync/syncable/syncable_id.h b/sync/syncable/syncable_id.h
index bbfeb9325d7e09893e71a29380729376c2e6ddda..a4995b1cff5e3a31b49c6070bc0eaeaa0c5473fe 100644
--- a/sync/syncable/syncable_id.h
+++ b/sync/syncable/syncable_id.h
@@ -43,9 +43,7 @@ SYNC_EXPORT_PRIVATE std::ostream& operator<<(std::ostream& out, const Id& id);
 // 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);
   }
@@ -63,14 +61,8 @@ class SYNC_EXPORT Id {
     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_);
   }
@@ -112,6 +104,9 @@ class SYNC_EXPORT Id {
   // 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,
@@ -124,8 +119,6 @@ class SYNC_EXPORT Id {
   std::string s_;
 };
 
-SYNC_EXPORT_PRIVATE Id GetNullId();
-
 }  // namespace syncable
 }  // namespace syncer