[SyncFS] Fix unexpected delay on running sync task

chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc

Index: chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc b/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc
index ab82ca252b3e0d69c78773d99accc9e32f94fbcb..d09a8430c527bd82330c0e1aa9b276dc809518af 100644
--- a/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc
+++ b/chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc
@@ -1000,9 +1000,11 @@ void MetadataDatabaseIndexOnDisk::UpdateInDirtyTrackerIndexes(
 
     DVLOG(1) << "  Remove from dirty trackers IDs: " << tracker_id;
 
+    if (DBHasKey(dirty_key))
+      --num_dirty_trackers_;
+
     db_->Delete(dirty_key);
     db_->Delete(demoted_key);
-    --num_dirty_trackers_;
   } else if (!old_tracker.dirty() && new_tracker.dirty()) {
     DCHECK(!DBHasKey(dirty_key));
     DCHECK(!DBHasKey(demoted_key));
@@ -1023,9 +1025,10 @@ void MetadataDatabaseIndexOnDisk::RemoveFromDirtyTrackerIndexes(
     DCHECK(DBHasKey(dirty_key) || DBHasKey(demoted_key));
 
     DVLOG(1) << "  Remove from dirty tracker IDs: " << tracker_id;
+    if (DBHasKey(dirty_key))
+      --num_dirty_trackers_;
     db_->Delete(dirty_key);
     db_->Delete(demoted_key);
-    --num_dirty_trackers_;
   }
 }
 
@@ -1137,12 +1140,6 @@ size_t MetadataDatabaseIndexOnDisk::CountDirtyTrackerInternal() const {
     ++num_dirty_trackers;
   }
 
-  for (itr->Seek(kDemotedDirtyIDKeyPrefix); itr->Valid(); itr->Next()) {
-    if (!StartsWithASCII(itr->key().ToString(), kDemotedDirtyIDKeyPrefix, true))
-      break;
-    ++num_dirty_trackers;
-  }
-
   return num_dirty_trackers;
 }