Add leakDetectionMetric for tracing

tracing/tracing/model/memory_dump_test_utils.html

Index: tracing/tracing/model/memory_dump_test_utils.html
diff --git a/tracing/tracing/model/memory_dump_test_utils.html b/tracing/tracing/model/memory_dump_test_utils.html
index aa0300e65d4742dd9f3197ca38fc85ee35d6a3fb..f322b48686d36d56ad0aad3bebc97b842b04db35 100644
--- a/tracing/tracing/model/memory_dump_test_utils.html
+++ b/tracing/tracing/model/memory_dump_test_utils.html
@@ -124,6 +124,7 @@ tr.exportTo('tr.model', function() {
         dump.addNumeric(numericName, castToScalar(value));
       }
     }
+    dump.addChildren(getOption(opt_options, 'children'));

[benjhayden, 2017/09/15 17:30:46]

MemoryAllocatorDump doesn't protect its children property, and newAllocatorDump
is test util, so I think it's ok for newAllocatorDump to set it directly instead
of adding a helper method to MemoryAllocatorDump:
const children = getOption(opt_options, 'children');
if (children) dump.children = children;

[yuzuchan, 2017/09/19 07:56:26]

Done.

     return dump;
   };