Add Metrics for heap high water marks

runtime/vm/metrics.cc

Index: runtime/vm/metrics.cc
diff --git a/runtime/vm/metrics.cc b/runtime/vm/metrics.cc
index 79038fb1f266f2588919773c14920df0d8401fc1..274a3fd179f5025f1a90f1adf032497eae0ee788 100644
--- a/runtime/vm/metrics.cc
+++ b/runtime/vm/metrics.cc
@@ -307,4 +307,30 @@ void Metric::Cleanup() {
   }
 }
 
+
+MaxMetric::MaxMetric()
+    : Metric() {
+  set_value(kMinInt64);
+}
+
+
+void MaxMetric::SetValue(int64_t new_value) {
+  if (new_value > value()) {
+    set_value(new_value);
+  }
+}
+
+
+MinMetric::MinMetric()
+    : Metric() {
+  set_value(kMaxInt64);
+}
+
+
+void MinMetric::SetValue(int64_t new_value) {
+  if (new_value < value()) {
+    set_value(new_value);
+  }
+}
+
 }  // namespace dart