Sync: Don't consider initial-sync-partially-done types as downloaded

DataTypeManagerImpl has the concept of "downloaded types", i.e. those
for which the initial sync (download+merge) has been completed.
Before this CL, types for which the initial sync was *partially* done
(which can only happen for ApplyUpdatesImmediatelyTypes(), in practice
only syncer::HISTORY) were considered downloaded.

This CL changes this so that (during startup) only fully-done types
are considered downloaded.

This has fairly little (but positive) practical impact: During startup,
if any partially-downloaded type exists, it'll trigger a configuration
cycle, and thus ensure that the download continues.

Bug: 1365938
Change-Id: Iabd4dbbc99435b53410b29fe0170f0e30fbc6f5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4352542


Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/main@{#1119477}