Update V8 to r6101 as required by WebKit r74534

Change-Id: I7f84af8dd732f11898fd644b2c2b1538914cb78d
diff --git a/src/vm-state.h b/src/vm-state.h
index cc91e83..df7fb30 100644
--- a/src/vm-state.h
+++ b/src/vm-state.h
@@ -36,34 +36,11 @@
 class VMState BASE_EMBEDDED {
 #ifdef ENABLE_VMSTATE_TRACKING
  public:
-  inline VMState(StateTag state);
+  inline explicit VMState(StateTag tag);
   inline ~VMState();
 
-  StateTag state() { return state_; }
-  void set_external_callback(Address external_callback) {
-    external_callback_ = external_callback;
-  }
-
-  // Used for debug asserts.
-  static bool is_outermost_external() {
-    return Top::current_vm_state() == 0;
-  }
-
-  static StateTag current_state() {
-    VMState* state = Top::current_vm_state();
-    return state ? state->state() : EXTERNAL;
-  }
-
-  static Address external_callback() {
-    VMState* state = Top::current_vm_state();
-    return state ? state->external_callback_ : NULL;
-  }
-
  private:
-  bool disabled_;
-  StateTag state_;
-  VMState* previous_;
-  Address external_callback_;
+  StateTag previous_tag_;
 
 #else
  public:
@@ -71,6 +48,20 @@
 #endif
 };
 
+
+class ExternalCallbackScope BASE_EMBEDDED {
+#ifdef ENABLE_LOGGING_AND_PROFILING
+ public:
+  inline explicit ExternalCallbackScope(Address callback);
+  inline ~ExternalCallbackScope();
+ private:
+  Address previous_callback_;
+#else
+ public:
+  explicit ExternalCallbackScope(Address callback) {}
+#endif
+};
+
 } }  // namespace v8::internal