Refactor histogram-set-view to an MVC pattern.

tracing/tracing/value/ui/histogram_set_view_state.html

Index: tracing/tracing/value/ui/histogram_set_view_state.html
diff --git a/tracing/tracing/value/ui/histogram_set_view_state.html b/tracing/tracing/value/ui/histogram_set_view_state.html
index 169845df0fc952a97a309b3629313e98de7c2a5f..2bd7d5a0a08d38aeee40b029eff854e0f7965fa7 100644
--- a/tracing/tracing/value/ui/histogram_set_view_state.html
+++ b/tracing/tracing/value/ui/histogram_set_view_state.html
@@ -21,7 +21,7 @@ tr.exportTo('tr.v.ui', function() {
       this.define('sortColumnIndex', 0);
       this.define('sortDescending', false);
       this.define('constrainNameColumn', true);
-      this.define('tableRowStates', []);
+      this.define('tableRowStates', new Map());
     }
   }
 
@@ -33,12 +33,12 @@ tr.exportTo('tr.v.ui', function() {
       this.define('isExpanded', false);
       this.define('isOverviewed', false);
       this.define('cells', new Map());
-      this.define('subRows', []);
+      this.define('subRows', new Map());
     }
 
     * walk() {
       yield this;
-      for (const row of this.subRows) yield* row.walk();
+      for (const row of this.subRows.values()) yield* row.walk();
     }
 
     static* walkAll(rootRows) {