sync: Better iteration in GenericChangeProcessor

This change introduces a new function for fetching the handles of all
children of a sync node, then puts it to use in optimizing the
GenericChangeProcessor's GetSyncDataForType() function.

Prior to the UniquePosition changes, it was simple and cheap to fetch
the ID of a successor or predecessor item.  After the change, it
requires a few expensive map lookups.  In other words, GetSuccessorId()
has gone from being O(1) to O(lg(n)).  This is a especially a problem in
code paths where we use GetSuccessorId() to iterate over all nodes in a
folder.

The UniquePosition change also makes it pretty easy to fetch all child
nodes under a given parent.  We could easily return all the EntryKernels
under a given folder.  Unfortunately, the APIs don't make it easy to
expose that functionality.  Instead, we do something less efficient, but
still much better than the status quo: return the IDs of all the
children.  The caller will need to look up each entry at O(lg(n)) cost,
but at least it didn't have to do two lookups to fetch each ID.

This change should lead to a slight performance improvement in the
ModelAssociation time of types that use the GenericChangeProcessor.

BUG=178275, 241813

Review URL: https://chromiumcodereview.appspot.com/14667013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202673 0039d316-1c4b-4281-b951-d872f2087c98

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