Add support for real-time audio threads.

base/threading/platform_thread.h

Index: base/threading/platform_thread.h
===================================================================
--- base/threading/platform_thread.h	(revision 84711)
+++ base/threading/platform_thread.h	(working copy)
@@ -47,6 +47,13 @@
 
 const PlatformThreadId kInvalidThreadId = 0;
 
+// Valid values for SetThreadPriority()
+enum ThreadPriority{
+  kThreadPriority_Normal,
+  // Suitable for low-latency, glitch-resistant audio.
+  kThreadPriority_RealtimeAudio
+};
+
 // A namespace for low-level thread functions.
 class BASE_API PlatformThread {
  public:
@@ -91,6 +98,9 @@
   // |thread_handle|.
   static void Join(PlatformThreadHandle thread_handle);
 
+  static void SetThreadPriority(PlatformThreadHandle handle,
+                                ThreadPriority priority);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformThread);
 };