[Sync] Get rid of SyncerError::SERVER_MORE_TO_DOWNLOAD

Unlike the rest SyncerErrors, SERVER_MORE_TO_DOWNLOAD has a very limited
scope close to GetUpdatesProcessor and doesn't really fit the enum.

This CL removes it by adding a simple getter to GetUpdatesProcessor and
reporting SYNCER_OK instead (only consumed by protocol event log in this
case, since there will be a follow up GetUpdatesRequest that will
overwrite SyncerError).

Bug: 1363089
Change-Id: If47a8ae224f33054582acc823e793c7d69d72fec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4513875
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Auto-Submit: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/main@{#1140891}