Implement resource-saving mode of Profiler.

src/log.h

Index: src/log.h
diff --git a/src/log.h b/src/log.h
index 6e5fb5bb7aa0a6d77598d3c8a85483118f0b6522..fc0b9f9a308e1237d8e955e644de3013688b4023 100644
--- a/src/log.h
+++ b/src/log.h
@@ -230,6 +230,9 @@ class Logger {
   // Logs a StringEvent regardless of whether FLAG_log is true.
   static void UncheckedStringEvent(const char* name, const char* value);
 
+  // Returns whether profiler's sampler is active.
+  static bool IsProfilerSamplerActive();
+
   // The sampler used by the profiler and the sliding state window.
   static Ticker* ticker_;
 
@@ -255,6 +258,8 @@ class Logger {
   friend class Profiler;
   friend class SlidingStateWindow;
   friend class VMState;
+
+  friend class LoggerTestHelper;
 #else
   static bool is_enabled() { return false; }
 #endif