Add full SetThreadPriority support to Windows.

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 7e6604b36fe0d33596c9ca98c4b957eefacd5c18..841a44a1b0b1cdc6b81443b1ebe990dbd6d9dbe7 100644
--- a/base/threading/platform_thread_internal_posix.cc
+++ b/base/threading/platform_thread_internal_posix.cc
@@ -20,6 +20,16 @@ int ThreadPriorityToNiceValue(ThreadPriority priority) {
   return 0;
 }
 
+ThreadPriority NiceValueToThreadPriority(int nice_value) {
+  for (const ThreadPriorityToNiceValuePair& pair :
+       kThreadPriorityToNiceValueMap) {
+    if (pair.nice_value == nice_value)
+      return pair.priority;
+  }
+  NOTREACHED() << "Unknown nice value";
+  return kThreadPriority_Normal;
+}
+
 }  // namespace internal
 
 }  // namespace base