Fix use of multiple raster threads for software rendering.

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 3cca2418bce076e4d4ada606f4575c992a1bef1e..35bd5e3f4e6f931153c499fdd4310855ce83febd 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -665,6 +665,8 @@ void RenderThreadImpl::Init() {
     DCHECK_GT(num_raster_threads, 0);
 
     // Force maximum 1 thread for threaded GPU rasterization.
+    // TODO(vmiura): crbug.com/459760 Support existence of multiple raster
+    // threads in GPU raster mode.
     if (is_threaded_gpu_rasterization_enabled_)
       num_raster_threads = 1;
     cc::TileTaskWorkerPool::SetNumWorkerThreads(num_raster_threads);