Fix compilation with clang 5.1 on OS X.

src/bootstrapper.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "bootstrapper.h"
 
 #include "accessors.h"
 #include "isolate-inl.h"
 #include "natives.h"
 #include "snapshot.h"
@@ skipping 2494 matching lines @@
 
 class NoTrackDoubleFieldsForSerializerScope {
  public:
   explicit NoTrackDoubleFieldsForSerializerScope(Isolate* isolate)
       : isolate_(isolate), flag_(FLAG_track_double_fields) {
     if (Serializer::enabled()) {
       // Disable tracking double fields because heap numbers treated as
       // immutable by the serializer.
       FLAG_track_double_fields = false;
     }
+    USE(isolate_);
   }
   ~NoTrackDoubleFieldsForSerializerScope() {
     if (Serializer::enabled()) {
       FLAG_track_double_fields = flag_;
     }
   }
 
  private:
   Isolate* isolate_;
   bool flag_;
@@ skipping 160 matching lines @@
   return from + sizeof(NestingCounterType);
 }
 
 
 // Called when the top-level V8 mutex is destroyed.
 void Bootstrapper::FreeThreadResources() {
   ASSERT(!IsActive());
 }
 
 } }  // namespace v8::internal