Dont send ADD for implicitly created app list folders during inital sync

Dont send ADD for implicitly created app list folders during inital sync

When processing the initial list of sync items, an item with parent_id
set to non-empty may trigger the creation of a new folder item, which
may trigger sending a sync ADD event for a sync item that already exists
(later in the list). The sync code treats an ADD request for an existing
item as a fatal error.

To prevent that, this CL disables sending ADD events while processing the
initial list; when the item is processed the newly created folder
SyncItem will be updated to reflect the initial sync list entry.

Note: An alternative approach would be to process the initial list twice,
creating folder items first then other items. Even though the change in
this CL is somewhat subtle, it is a much smaller change than processing
the list twice twice would be, and I think that processing the list twice
would  ultimately be more error prone. That said, I could be convinced
otherwise.

BUG=380537

Review URL: https://codereview.chromium.org/580653002

Cr-Commit-Position: refs/heads/master@{#295300}