Improvements to uses of base::SmallMap

cc/tiles/picture_layer_tiling.h

Index: cc/tiles/picture_layer_tiling.h
diff --git a/cc/tiles/picture_layer_tiling.h b/cc/tiles/picture_layer_tiling.h
index ecf1974efa810a069b0dd9792648fd119ba16e39..a74a058c32f2233e6bfd0ea99847e0c4ace7b825 100644
--- a/cc/tiles/picture_layer_tiling.h
+++ b/cc/tiles/picture_layer_tiling.h
@@ -63,6 +63,9 @@ struct TileMapKey {
   bool operator==(const TileMapKey& other) const {
     return index_x == other.index_x && index_y == other.index_y;
   }
+  bool operator<(const TileMapKey& other) const {
+    return std::tie(index_x, index_y) < std::tie(other.index_x, other.index_y);
+  }
 
   int index_x;
   int index_y;