Fix NiceValueToThreadPriority failing on an unknown nice value.

base/threading/platform_thread_internal_posix.cc

Index: base/threading/platform_thread_internal_posix.cc
diff --git a/base/threading/platform_thread_internal_posix.cc b/base/threading/platform_thread_internal_posix.cc
index 9af02044fceed2635b4fc5bf1ee5e1a06927a6fe..378a24d0d1d8f9be8aed6da1b6a2dca7ce9dd641 100644
--- a/base/threading/platform_thread_internal_posix.cc
+++ b/base/threading/platform_thread_internal_posix.cc
@@ -4,6 +4,7 @@
 
 #include "base/threading/platform_thread_internal_posix.h"
 
+#include "base/containers/adapters.h"
 #include "base/logging.h"
 
 namespace base {
@@ -11,8 +12,7 @@ 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;
   }
@@ -21,13 +21,17 @@ int ThreadPriorityToNiceValue(ThreadPriority priority) {
 }
 
 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