Increase priority of raster threads on Android.

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 3468f45f91efd4929ea838f91559251a2d75a4ef..26d5e2da4513ab885476c9ac57b8c66fb92a8eb4 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -115,8 +115,11 @@ 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).