[runtime-call-stats] Make sure GCTracer::Scope makes paired Enter/Leave calls

Ensure that RuntimeCallStats::Enter is paired with Leave when
FLAG_runtime_stats changes in flight.

BUG=chromium:669329

Change-Id: I4da7edf88990fdebd7d05325a09cfca0702cfe5a
Reviewed-on: https://chromium-review.googlesource.com/643472Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47749}

src/heap/gc-tracer.cc

@@ -32,21 +32,18 @@ GCTracer::Scope::Scope(GCTracer* tracer, ScopeId scope)
     : tracer_(tracer), scope_(scope) {
   start_time_ = tracer_->heap_->MonotonicallyIncreasingTimeInMs();
   // TODO(cbruni): remove once we fully moved to a trace-based system.
-  if (V8_UNLIKELY(FLAG_runtime_stats)) {
-    RuntimeCallStats::Enter(
-        tracer_->heap_->isolate()->counters()->runtime_call_stats(), &timer_,
+  if (V8_LIKELY(!FLAG_runtime_stats)) return;
+  runtime_stats_ = tracer_->heap_->isolate()->counters()->runtime_call_stats();
+  RuntimeCallStats::Enter(runtime_stats_, &timer_,
                           GCTracer::RCSCounterFromScope(scope));
-  }
 }
 
 GCTracer::Scope::~Scope() {
   tracer_->AddScopeSample(
       scope_, tracer_->heap_->MonotonicallyIncreasingTimeInMs() - start_time_);
   // TODO(cbruni): remove once we fully moved to a trace-based system.
-  if (V8_UNLIKELY(FLAG_runtime_stats)) {
-    RuntimeCallStats::Leave(
-        tracer_->heap_->isolate()->counters()->runtime_call_stats(), &timer_);
-  }
+  if (V8_LIKELY(runtime_stats_ == nullptr)) return;
+  RuntimeCallStats::Leave(runtime_stats_, &timer_);
 }
 
 const char* GCTracer::Scope::Name(ScopeId id) {

src/heap/gc-tracer.h

@@ -81,6 +81,7 @@ class V8_EXPORT_PRIVATE GCTracer {
     ScopeId scope_;
     double start_time_;
     RuntimeCallTimer timer_;
+    RuntimeCallStats* runtime_stats_ = nullptr;
 
     DISALLOW_COPY_AND_ASSIGN(Scope);
   };