Fix NiceValueToThreadPriority failing on an unknown nice value.

base/threading/platform_thread_internal_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/threading/platform_thread_internal_posix.h"
 
+#include "base/containers/adapters.h"
 #include "base/logging.h"
 
 namespace base {
 
 namespace internal {
 
 int ThreadPriorityToNiceValue(ThreadPriority priority) {
-  for (const ThreadPriorityToNiceValuePair& pair :
-       kThreadPriorityToNiceValueMap) {
+  for (const auto& pair : kThreadPriorityToNiceValueMap) {
     if (pair.priority == priority)
       return pair.nice_value;
   }
   NOTREACHED() << "Unknown ThreadPriority";
   return 0;
 }
 
 ThreadPriority NiceValueToThreadPriority(int nice_value) {
-  for (const ThreadPriorityToNiceValuePair& pair :
-       kThreadPriorityToNiceValueMap) {
-    if (pair.nice_value == nice_value)
+  // Try to find a priority that best describes |nice_value|. If there isn't
+  // an exact match, this method returns the closest priority whose nice value
+  // is higher (lower priority) than |nice_value|.
+  for (const auto& pair : Reversed(kThreadPriorityToNiceValueMap)) {
+    if (pair.nice_value >= nice_value)
       return pair.priority;
   }
-  NOTREACHED() << "Unknown nice value";
-  return ThreadPriority::NORMAL;
+
+  // Reaching here means |nice_value| is more than any of the defined
+  // priorities. The lowest priority is suitable in this case.
+  return ThreadPriority::BACKGROUND;
 }
 
 }  // namespace internal
 
 }  // namespace base