Fix NiceValueToThreadPriority failing on an unknown nice value.

base/threading/platform_thread_internal_posix.h

Index: base/threading/platform_thread_internal_posix.h
diff --git a/base/threading/platform_thread_internal_posix.h b/base/threading/platform_thread_internal_posix.h
index 05a8d1e26e78bd56d4e539110aad330a4ed01b1d..5f4a215968b60dbd61f067280bd958364b498353 100644
--- a/base/threading/platform_thread_internal_posix.h
+++ b/base/threading/platform_thread_internal_posix.h
@@ -5,6 +5,7 @@
 #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 {
@@ -15,7 +16,11 @@ struct ThreadPriorityToNiceValuePair {
   ThreadPriority priority;
   int nice_value;
 };
-extern const ThreadPriorityToNiceValuePair kThreadPriorityToNiceValueMap[4];
+// 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).
+BASE_EXPORT extern
+const ThreadPriorityToNiceValuePair kThreadPriorityToNiceValueMap[4];
 
 // Returns the nice value matching |priority| based on the platform-specific
 // implementation of kThreadPriorityToNiceValueMap.
@@ -23,7 +28,7 @@ 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