Implement resource-saving mode of Profiler.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index e23abfc375de08380991ea1dee918b45298f3bf2..3c2318f64e7f1d61316e1f702c24074426ab6ad7 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -518,10 +518,11 @@ class Sampler {
   // Is the sampler used for profiling.
   inline bool IsProfiling() { return profiling_; }
 
-  class PlatformData;
- protected:
+  // Whether the sampler is running (that is, consumes resources).
   inline bool IsActive() { return active_; }
 
+  class PlatformData;
+
  private:
   int interval_;
   bool profiling_;