base: Support setting thread priorities generically.

base/threading/platform_thread.h

Index: base/threading/platform_thread.h
diff --git a/base/threading/platform_thread.h b/base/threading/platform_thread.h
index 576695a4e7b5f8028f47cc5e02921b551ffb0a39..ab897bbda6642653c26068dbcee5c7c1fd337b9e 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -43,7 +43,11 @@ const PlatformThreadId kInvalidThreadId = 0;
 enum ThreadPriority{
   kThreadPriority_Normal,
   // Suitable for low-latency, glitch-resistant audio.
-  kThreadPriority_RealtimeAudio
+  kThreadPriority_RealtimeAudio,
+  // Suitable for threads which generate data for the display (at ~60Hz).
+  kThreadPriority_Display,
+  // Suitable for threads that shouldn't disrupt high priority work.
+  kThreadPriority_Background
 };
 
 // A namespace for low-level thread functions.
@@ -106,9 +110,10 @@ class BASE_EXPORT PlatformThread {
   // |thread_handle|.
   static void Join(PlatformThreadHandle thread_handle);
 
-  // Sets the priority of the thread specified in |handle| to |priority|.
-  // This does not work on Linux, use CreateWithPriority() instead.
+  // Only one of the handle/id is required depending on the
+  // platform. However, for cross platform code, pass both.
   static void SetThreadPriority(PlatformThreadHandle handle,
+                                PlatformThreadId id,
                                 ThreadPriority priority);
 
  private: