Use a common buffer across all instances for stack-copy.

base/profiler/native_stack_sampler_posix.cc

 // Copyright 2015 The Chromium 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 "base/profiler/native_stack_sampler.h"
 
 namespace base {
 
 std::unique_ptr<NativeStackSampler> NativeStackSampler::Create(
     PlatformThreadId thread_id,
     AnnotateCallback annotator,
     NativeStackSamplerTestDelegate* test_delegate) {
   return std::unique_ptr<NativeStackSampler>();
 }
 
+std::unique_ptr<NativeStackSampler::StackBuffer>
+NativeStackSampler::CreateStackBuffer() {
+  return nullptr;
+}
+
 }  // namespace base