Enable Null Syncable ID which is different than Root ID.

This patch prepares sync codebase for supporting datatypes with
implicit permanent forlders by introducing an empty/unset
syncable ID.
There was a notion of Null ID before but it was equivalent
to the root ID which caused some confusion in the code - a Null
ID would sometimes be used where a "root" would be appropriate
and vice versa.
See the following code fragment for an example:
// 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();
}

This change will be followed by one or two separate changes
that will actually remove dependencies on PARENT_ID and
SERVER_PARENT_ID and use a different way to organize nodes
in the directory based on node's datatype rather than parent ID.

This change impacts the following:
1) The default ID value in EntryKernel ID fields and elsewhere is
Null ID and not Root ID as it used to be.
2) This means that some tests that depended on Root ID being
set implicitly
in PARENT_ID and SERVER_PARENT_ID now need to set these IDs explicitly.
3) Node's GetPredecessorId, GetSuccessorId, and GetFirstChildId
methods now return a null ID rather than a root ID when there
is no predecessor, successor, or children.
4) Small changed throughout the code to make sure that
null IDs and root IDs are used accordingly (now that they
are no longer equivalent).
5) Changed some validation methods to accept Null IDs
to avoid large changes in the test code.

BUG=438313

Review URL: https://codereview.chromium.org/805633004

Cr-Commit-Position: refs/heads/master@{#309495}

1 files changed, 3 insertions, 2 deletions

diff --git a/sync/syncable/directory.cc b/sync/syncable/directory.cc
index 9c59409..22c3f46 100644
--- a/sync/syncable/directory.cc
+++ b/sync/syncable/directory.cc
@@ -1192,6 +1192,7 @@ bool Directory::CheckTreeInvariants(syncable::BaseTransaction* trans,
                       trans))
         return false;
       int safety_count = handles.size() + 1;
+      // TODO(stanisc): handle items with Null parentid
       while (!parentid.IsRoot()) {
         Entry parent(trans, GET_BY_ID, parentid);
         if (!SyncAssert(parent.good(), FROM_HERE,
@@ -1392,13 +1393,13 @@ void Directory::PutPredecessor(EntryKernel* e, EntryKernel* predecessor) {
 
   if (!siblings) {
     // This parent currently has no other children.
-    DCHECK(predecessor->ref(ID).IsRoot());
+    DCHECK(predecessor == NULL);
     UniquePosition pos = UniquePosition::InitialPosition(suffix);
     e->put(UNIQUE_POSITION, pos);
     return;
   }
 
-  if (predecessor->ref(ID).IsRoot()) {
+  if (predecessor == NULL) {
     // We have at least one sibling, and we're inserting to the left of them.
     UniquePosition successor_pos = (*siblings->begin())->ref(UNIQUE_POSITION);