Fix NiceValueToThreadPriority failing on an unknown nice value.

base/threading/platform_thread_internal_posix.h

 // 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.
 
 #ifndef BASE_THREADING_PLATFORM_THREAD_INTERNAL_POSIX_H_
 #define BASE_THREADING_PLATFORM_THREAD_INTERNAL_POSIX_H_
 
+#include "base/base_export.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 
 namespace internal {
 
 struct ThreadPriorityToNiceValuePair {
   ThreadPriority priority;
   int nice_value;
 };
+// The elements must be listed in the order of increasing priority (lowest
+// priority first), that is, in the order of decreasing nice values (highest
+// nice value first).
 extern const ThreadPriorityToNiceValuePair kThreadPriorityToNiceValueMap[4];
 
 // Returns the nice value matching |priority| based on the platform-specific
 // implementation of kThreadPriorityToNiceValueMap.
 int ThreadPriorityToNiceValue(ThreadPriority priority);
 
 // Returns the ThreadPrioirty matching |nice_value| based on the platform-
 // specific implementation of kThreadPriorityToNiceValueMap.
-ThreadPriority NiceValueToThreadPriority(int nice_value);
+BASE_EXPORT ThreadPriority NiceValueToThreadPriority(int nice_value);
 
 // Allows platform specific tweaks to the generic POSIX solution for
 // SetCurrentThreadPriority. Returns true if the platform-specific
 // implementation handled this |priority| change, false if the generic
 // implementation should instead proceed.
 bool SetCurrentThreadPriorityForPlatform(ThreadPriority priority);
 
 // Returns true if there is a platform-specific ThreadPriority set on the
 // current thread (and returns the actual ThreadPriority via |priority|).
 // Returns false otherwise, leaving |priority| untouched.
 bool GetCurrentThreadPriorityForPlatform(ThreadPriority* priority);
 
 }  // namespace internal
 
 }  // namespace base
 
 #endif  // BASE_THREADING_PLATFORM_THREAD_INTERNAL_POSIX_H_