Update from https://crrev.com/327068

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..653961ddfae05b34ddbf3def5fbe91f20e523c51 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -110,15 +110,17 @@ class PlatformThreadHandle {
 
 const PlatformThreadId kInvalidThreadId(0);
 
-// Valid values for SetThreadPriority()
-enum ThreadPriority{
-  kThreadPriority_Normal,
-  // Suitable for low-latency, glitch-resistant audio.
-  kThreadPriority_RealtimeAudio,
-  // Suitable for threads which generate data for the display (at ~60Hz).
-  kThreadPriority_Display,
+// Valid values for SetThreadPriority(), listed in increasing order of
+// importance.
+enum class ThreadPriority {
   // Suitable for threads that shouldn't disrupt high priority work.
-  kThreadPriority_Background
+  BACKGROUND,
+  // Default priority level.
+  NORMAL,
+  // Suitable for threads which generate data for the display (at ~60Hz).
+  DISPLAY,
+  // Suitable for low-latency, glitch-resistant audio.
+  REALTIME_AUDIO,
 };
 
 // A namespace for low-level thread functions.
@@ -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 Windows, this is 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);
 };