Add leakDetectionMetric for tracing

tracing/tracing/model/memory_allocator_dump.html

Index: tracing/tracing/model/memory_allocator_dump.html
diff --git a/tracing/tracing/model/memory_allocator_dump.html b/tracing/tracing/model/memory_allocator_dump.html
index 42174194ebfdce7a16f2e109697818b23f136030..26e9fa59ffe58d8325f10630bcd12559d1105c9a 100644
--- a/tracing/tracing/model/memory_allocator_dump.html
+++ b/tracing/tracing/model/memory_allocator_dump.html
@@ -112,6 +112,12 @@ tr.exportTo('tr.model', function() {
       this.diagnostics[name] = text;
     },
 
+    addChildren(array) {
+      if (Array.isArray(array)) {
+        this.children = array;
+      }
+    },
+
     aggregateNumericsRecursively(opt_model) {
       const numericNames = new Set();