[sync] Fix bookmarks last-sync-time not being persisted

crrev.com/c/2296420 recently introduced some heuristics that use the
notion of when bookmarks were sync-ed for the last time. In such patch,
the persistence of this time was overlooked, fixed here.

Change-Id: I9eb5046e1dcae1affb99fe2fc732678c7d922dce
Bug: 1032052
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2306033
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Auto-Submit: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789880}

components/sync_bookmarks/synced_bookmark_tracker.cc

@@ -445,6 +445,7 @@ SyncedBookmarkTracker::BuildBookmarkModelMetadata() const {
   sync_pb::BookmarkModelMetadata model_metadata;
   model_metadata.set_bookmarks_full_title_reuploaded(
       bookmarks_full_title_reuploaded_);
+  model_metadata.set_last_sync_time(syncer::TimeToProtoTime(last_sync_time_));
   for (const std::pair<const std::string, std::unique_ptr<Entity>>& pair :
        sync_id_to_entities_map_) {
     DCHECK(pair.second) << " for ID " << pair.first;

components/sync_bookmarks/synced_bookmark_tracker_unittest.cc

@@ -186,6 +186,7 @@ TEST(SyncedBookmarkTrackerTest, ShouldBuildBookmarkModelMetadata) {
   EXPECT_THAT(
       bookmark_model_metadata.bookmarks_metadata(0).metadata().server_id(),
       Eq(kSyncId));
+  EXPECT_TRUE(bookmark_model_metadata.has_last_sync_time());
 }
 
 TEST(SyncedBookmarkTrackerTest,