Revert r14252 as it broke --prof for some cases

src/sampler.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 33 matching lines @@
 // the thread that created it.
 
 // TickSample captures the information collected for each sample.
 struct TickSample {
   TickSample()
       : state(OTHER),
         pc(NULL),
         sp(NULL),
         fp(NULL),
         external_callback(NULL),
-        frames_count(0) {}
+        frames_count(0),
+        has_external_callback(false) {}
   void Trace(Isolate* isolate);
   StateTag state;  // The state of the VM.
   Address pc;      // Instruction pointer.
   Address sp;      // Stack pointer.
   Address fp;      // Frame pointer.
-  Address external_callback;
+  union {
+    Address tos;   // Top stack value (*sp).
+    Address external_callback;
+  };
   static const int kMaxFramesCount = 64;
   Address stack[kMaxFramesCount];  // Call stack.
   int frames_count : 8;  // Number of captured frames.
+  bool has_external_callback : 1;
 };
 
 class Sampler {
  public:
   // Initializes the Sampler support. Called once at VM startup.
   static void SetUp();
   static void TearDown();
 
   // Initialize sampler.
   Sampler(Isolate* isolate, int interval);
@@ skipping 37 matching lines @@
   Atomic32 active_;
   PlatformData* data_;  // Platform specific data.
   int samples_taken_;  // Counts stack samples taken.
   DISALLOW_IMPLICIT_CONSTRUCTORS(Sampler);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SAMPLER_H_