Perform CPU sampling by CPU sampling thread only iff processing thread is not running.

src/cpu-profiler.cc

Index: src/cpu-profiler.cc
diff --git a/src/cpu-profiler.cc b/src/cpu-profiler.cc
index 8f72c1755cb898264e4d2a5ba029ebc77b019f09..220daca14b18dd8436c0b166cf6ebee1966fd208 100644
--- a/src/cpu-profiler.cc
+++ b/src/cpu-profiler.cc
@@ -250,9 +250,7 @@ void ProfilerEventsProcessor::Run() {
 
   while (running_) {
     int64_t stop_time = OS::Ticks() + period_in_useconds_;
-    if (sampler_ != NULL) {
-      sampler_->DoSample();
-    }
+    sampler_->DoSample();
     ProcessEventsQueue(stop_time, &dequeue_order);
   }
 
@@ -519,6 +517,7 @@ void CpuProfiler::StartProcessorIfNotStarted() {
       sampler->Start();
       need_to_stop_sampler_ = true;
     }
+    sampler->SetHasProcessingThread(true);
     sampler->IncreaseProfilingDepth();
     processor_->Start();
   }
@@ -561,6 +560,7 @@ void CpuProfiler::StopProcessor() {
   Logger* logger = Isolate::Current()->logger();
   Sampler* sampler = logger->sampler();
   sampler->DecreaseProfilingDepth();
+  sampler->SetHasProcessingThread(false);
   if (need_to_stop_sampler_) {
     sampler->Stop();
     need_to_stop_sampler_ = false;