[counters] RuntimeStats: fix wrong bookkeeping when dynamically changing counters.

src/counters-inl.h

Index: src/counters-inl.h
diff --git a/src/counters-inl.h b/src/counters-inl.h
index 7219ef778a6ea6a06ce25ad457b7a1299c5d7194..066a827a853f9620720089ec8c691067d504348a 100644
--- a/src/counters-inl.h
+++ b/src/counters-inl.h
@@ -10,6 +10,33 @@
 namespace v8 {
 namespace internal {
 
+void RuntimeCallTimer::Start(RuntimeCallCounter* counter,
+                             RuntimeCallTimer* parent) {
+  counter_ = counter;
+  parent_.SetValue(parent);
+  if (FLAG_runtime_stats !=
+      v8::tracing::TracingCategoryObserver::ENABLED_BY_SAMPLING) {
+    timer_.Start();
+  }
+}
+
+RuntimeCallTimer* RuntimeCallTimer::Stop() {
+  if (!timer_.IsStarted()) return parent();
+  base::TimeDelta delta = timer_.Elapsed();
+  timer_.Stop();
+
+  counter_->Increment();
+  AddAndSubmitResults(delta);
+
+  RuntimeCallTimer* parent_timer = parent();
+  if (parent_timer) {
+    // Subtract the elapsed time from the parent to correctly calculate its own
+    // time.
+    parent_timer->SubtractSubtime(delta);
+  }
+  return parent_timer;
+}
+
 RuntimeCallTimerScope::RuntimeCallTimerScope(
     Isolate* isolate, RuntimeCallStats::CounterId counter_id) {
   if (V8_UNLIKELY(FLAG_runtime_stats)) {