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

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index 4b558df172385085d76a40b4efe6cc2a0e673373..d4c91b4a1db1e05f3409c945b70f1af879ac30ef 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -760,10 +760,28 @@ class Sampler {
   void Start();
   void Stop();
 
-  // Is the sampler used for profiling?
-  bool IsProfiling() const { return NoBarrier_Load(&profiling_) > 0; }
-  void IncreaseProfilingDepth() { NoBarrier_AtomicIncrement(&profiling_, 1); }
-  void DecreaseProfilingDepth() { NoBarrier_AtomicIncrement(&profiling_, -1); }
+  // Whether the sampling thread should use this Sampler for CPU profiling?
+  bool IsProfiling() const {
+    return NoBarrier_Load(&profiling_) > 0 &&
+        !NoBarrier_Load(&has_processing_thread_);
+  }
+  // Perform platform-specific initialization before DoSample() may be invoked.
+  void StartSampling();
+  // Perform platform-specific cleanup after samping.
+  void StopSampling();
+  void IncreaseProfilingDepth() {
+    if (NoBarrier_AtomicIncrement(&profiling_, 1) == 1) {
+      StartSampling();
+    }
+  }
+  void DecreaseProfilingDepth() {
+    if (!NoBarrier_AtomicIncrement(&profiling_, -1)) {
+      StopSampling();
+    }
+  }
+  void SetHasProcessingThread(bool value) {
+    NoBarrier_Store(&has_processing_thread_, value);
+  }
 
   // Whether the sampler is running (that is, consumes resources).
   bool IsActive() const { return NoBarrier_Load(&active_); }
@@ -790,6 +808,7 @@ class Sampler {
   const int interval_;
   Atomic32 profiling_;
   Atomic32 active_;
+  Atomic32 has_processing_thread_;
   PlatformData* data_;  // Platform specific data.
   int samples_taken_;  // Counts stack samples taken.
   DISALLOW_IMPLICIT_CONSTRUCTORS(Sampler);