content: Implement dynamic priorities for raster threads.

base/threading/platform_thread_internal_posix.h

Index: base/threading/platform_thread_internal_posix.h
diff --git a/base/threading/platform_thread_internal_posix.h b/base/threading/platform_thread_internal_posix.h
index 05a8d1e26e78bd56d4e539110aad330a4ed01b1d..a3f815ada26c102f4b9b70708601d07c54a4e56b 100644
--- a/base/threading/platform_thread_internal_posix.h
+++ b/base/threading/platform_thread_internal_posix.h
@@ -31,6 +31,10 @@ ThreadPriority NiceValueToThreadPriority(int nice_value);
 // implementation should instead proceed.
 bool SetCurrentThreadPriorityForPlatform(ThreadPriority priority);
 
+// Sets plaftorm specific |priority| to given |tid|.
+bool SetChildThreadPriorityInCurrentProcess(PlatformThreadId tid,
+                                            ThreadPriority priority);
+
 // Returns true if there is a platform-specific ThreadPriority set on the
 // current thread (and returns the actual ThreadPriority via |priority|).
 // Returns false otherwise, leaving |priority| untouched.