Update from https://crrev.com/333737

base/threading/platform_thread.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // WARNING: You should *NOT* be using this class directly.  PlatformThread is
 // the low-level platform-specific abstraction to the OS's threading interface.
 // You should instead be using a message-loop driven Thread, see thread.h.
 
 #ifndef BASE_THREADING_PLATFORM_THREAD_H_
 #define BASE_THREADING_PLATFORM_THREAD_H_
@@ skipping 71 matching lines @@
       : handle_(handle),
         id_(0) {
   }
 
   PlatformThreadHandle(Handle handle,
                        PlatformThreadId id)
       : handle_(handle),
         id_(id) {
   }
 
+  PlatformThreadId id() const {
+    return id_;
+  }
+
   bool is_equal(const PlatformThreadHandle& other) const {
     return handle_ == other.handle_;
   }
 
   bool is_null() const {
     return !handle_;
   }
 
   Handle platform_handle() const {
     return handle_;
   }
 
  private:
-  friend class PlatformThread;
-
   Handle handle_;
   PlatformThreadId id_;
 };
 
 const PlatformThreadId kInvalidThreadId(0);
 
 // Valid values for SetThreadPriority(), listed in increasing order of
 // importance.
 enum class ThreadPriority {
   // Suitable for threads that shouldn't disrupt high priority work.
@@ skipping 66 matching lines @@
   // CreateNonJoinable() does the same thing as Create() except the thread
   // cannot be Join()'d.  Therefore, it also does not output a
   // PlatformThreadHandle.
   static bool CreateNonJoinable(size_t stack_size, Delegate* delegate);
 
   // Joins with a thread created via the Create function.  This function blocks
   // the caller until the designated thread exits.  This will invalidate
   // |thread_handle|.
   static void Join(PlatformThreadHandle thread_handle);
 
+  // Toggles the target thread's priority at runtime.  Prefer
+  // CreateWithPriority() to set the thread's initial priority.
+  // NOTE: The call may fail if the caller thread is not the same as the
+  // target thread on POSIX.  For example, seccomp-bpf blocks it by default
+  // in the sandbox.
   static void SetThreadPriority(PlatformThreadHandle handle,
                                 ThreadPriority priority);
 
   static ThreadPriority GetThreadPriority(PlatformThreadHandle handle);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformThread);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_PLATFORM_THREAD_H_