Add "UNKNOWN_TILE_TYPE" for metrics

iOS doesn't yet have any delegate method that fires when we have
finished loading all the tiles on the NTP. Without that, it's not
possible to reliably output metrics for tile types. Adding this (after
NUM_RECORDED_TILE_TYPES) will let us proceed with the conversion to
metrics::RecordPageImpression() in the meantime.

BUG=669090

Review-Url: https://codereview.chromium.org/2539473002
Cr-Commit-Position: refs/heads/master@{#434985}

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.