Increase priority of raster threads on Android.

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 97 matching lines @@
  private:
   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.
+  // For threads that should be throttled under contention.
   BACKGROUND,
+  // For threads that shouldn't disrupt higher-priority work, but which produce
+  // user-visible results.
+  UI_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.
 class BASE_EXPORT PlatformThread {
@@ skipping 72 matching lines @@
 
   static ThreadPriority GetThreadPriority(PlatformThreadHandle handle);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformThread);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_PLATFORM_THREAD_H_