[Sync] Add HasChildren() function and use it instead of GetFirstChildId()

chrome/browser/sync/syncable/syncable.h

Index: chrome/browser/sync/syncable/syncable.h
diff --git a/chrome/browser/sync/syncable/syncable.h b/chrome/browser/sync/syncable/syncable.h
index 4f8875f92ddac46987a8a898a70f446fbde1fc8c..6bf46e5aaa2834cba713f76bc8a64a88f039f21f 100644
--- a/chrome/browser/sync/syncable/syncable.h
+++ b/chrome/browser/sync/syncable/syncable.h
@@ -920,9 +920,12 @@ class Directory {
   void GetChildHandlesByHandle(BaseTransaction*, int64 handle,
       ChildHandles* result);
 
+  // Returns true iff |id| has children.

[rlarocque, 2011/10/26 22:43:18]

Should we mention that self-looped children are not counted as children?  

That's an honest question.  I have no idea what it means for a child to be
self-looped, or what a caller of this function might expect if there are only
self-looped children.

[akalin, 2011/10/27 00:19:21]

I don't think so, since deleted == self-looped.

However, the two functions above, which do return self-looped nodes, should be
documented as such.  Done.

+  bool HasChildren(BaseTransaction* trans, const Id& id);
+
   // Find the first or last child in the positional ordering under a parent,
   // and return its id.  Returns a root Id if parent has no children.
-  virtual Id GetFirstChildId(BaseTransaction* trans, const Id& parent_id);
+  Id GetFirstChildId(BaseTransaction* trans, const Id& parent_id);
   Id GetLastChildId(BaseTransaction* trans, const Id& parent_id);
 
   // Compute a local predecessor position for |update_item|.  The position
@@ -1149,6 +1152,12 @@ class Directory {
       const ScopedKernelLock& lock,
       const Id& parent_id, Directory::ChildHandles* result);
 
+  // Returns a pointer to what is probably (but not certainly) the
+  // first child of |parent_id|, or NULL if |parent_id| definitely has
+  // no children.
+  EntryKernel* GetPossibleFirstChild(
+      const ScopedKernelLock& lock, const Id& parent_id);
+
   Kernel* kernel_;
 
   DirectoryBackingStore* store_;