Add "UNKNOWN_TILE_TYPE" for metrics

components/ntp_tiles/metrics.h

Index: components/ntp_tiles/metrics.h
diff --git a/components/ntp_tiles/metrics.h b/components/ntp_tiles/metrics.h
index 5fcb8168239de20de61bc41350e0ab27ea8d1c67..c32dbf258df3a21c54958841bd61274cf2120c98 100644
--- a/components/ntp_tiles/metrics.h
+++ b/components/ntp_tiles/metrics.h
@@ -34,6 +34,9 @@ enum MostVisitedTileType {
   NUM_RECORDED_TILE_TYPES,
   // The item displays a thumbnail of the page. Used on desktop.
   THUMBNAIL,
+  // The tile type has not been determined yet. Used on iOS, until we can detect
+  // when all tiles have loaded.
+  UNKNOWN_TILE_TYPE,
 };
 
 // Records an NTP impression, after all tiles have loaded.