base: Support setting thread priorities generically.

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index cf1a86da4d39e374c066c7c090dd14b8c17ae7d2..15f73a02ea6d960ffc6dd9cebeedfbe7f63db565 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -142,6 +142,9 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // Returns true if the thread has been started, and not yet stopped.
   bool IsRunning() const;
 
+  // Sets the thread priority. The thread must already be started.
+  void SetPriority(ThreadPriority priority);
+
  protected:
   // Called just prior to starting the message loop
   virtual void Init() {}