Add full SetThreadPriority support to Windows.

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 28743145610c70fe14f8c8fdd1e377a9aa861528..5d0c537bb843084fc089f7a3e7ed34e27aa5a25a 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -111,13 +111,15 @@ class PlatformThreadHandle {
 const PlatformThreadId kInvalidThreadId(0);
 
 // Valid values for SetThreadPriority()
-enum ThreadPriority{
+enum ThreadPriority {
   kThreadPriority_Normal,

[rvargas (doing something else), 2015/03/19 22:19:21]

Let's order this according to priority.

[gab, 2015/03/30 20:14:45]

I was thinking about that too, but I was thinking we should probably also make
this an "enum class" (since we toggle it back-and-forth with the OS priority
expressed as an integer and can easily mix the two otherwise) -- feels like this
belongs in a follow-up (or precursor CL), WDYT?

[rvargas (doing something else), 2015/03/30 22:31:22]

I doesn't have to be part of this CL, but we need a clear priority order here.

   // Suitable for low-latency, glitch-resistant audio.
   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.
+  // Suitable for threads that shouldn't disrupt high priority work. Extra
+  // background properties (low IO/memory priority) can be achieved on some
+  // platforms if this mode is set from the current thread.

[rvargas (doing something else), 2015/03/19 22:19:21]

nit: "if this mode"? Do you mean this priority?

[gab, 2015/03/30 20:14:45]

Done.

   kThreadPriority_Background
 };
 
@@ -141,7 +143,10 @@ class BASE_EXPORT PlatformThread {
   // we're on the right thread quickly.
   static PlatformThreadRef CurrentRef();
 
-  // Get the current handle.
+  // Get the handle representing the current thread. On some platforms, this can
+  // be a pseudo handle constant which will always represent the thread using it
+  // and hence should not be shared with other threads nor be used to
+  // differentiate the current thread from another.
   static PlatformThreadHandle CurrentHandle();
 
   // Yield the current thread so another thread can be scheduled.
@@ -171,9 +176,7 @@ class BASE_EXPORT PlatformThread {
 
   // CreateWithPriority() does the same thing as Create() except the priority of
   // the thread is set based on |priority|.  Can be used in place of Create()
-  // followed by SetThreadPriority().  SetThreadPriority() has not been
-  // implemented on the Linux platform yet, this is the only way to get a high
-  // priority thread on Linux.
+  // followed by SetThreadPriority().
   static bool CreateWithPriority(size_t stack_size, Delegate* delegate,
                                  PlatformThreadHandle* thread_handle,
                                  ThreadPriority priority);
@@ -191,6 +194,8 @@ class BASE_EXPORT PlatformThread {
   static void SetThreadPriority(PlatformThreadHandle handle,
                                 ThreadPriority priority);
 
+  static ThreadPriority GetThreadPriority(PlatformThreadHandle handle);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformThread);
 };