Replaced is_null() with is_valid in SurfaceId and related classes.

cc/surfaces/direct_compositor_frame_sink.cc

Index: cc/surfaces/direct_compositor_frame_sink.cc
diff --git a/cc/surfaces/direct_compositor_frame_sink.cc b/cc/surfaces/direct_compositor_frame_sink.cc
index 194a08e7be50f4de6ca595b1e47dec2cd2979f25..accfe36a4ff2b1769e1aa560acc8955328122059 100644
--- a/cc/surfaces/direct_compositor_frame_sink.cc
+++ b/cc/surfaces/direct_compositor_frame_sink.cc
@@ -78,7 +78,7 @@ void DirectCompositorFrameSink::DetachFromClient() {
   // Unregister the SurfaceFactoryClient here instead of the dtor so that only
   // one client is alive for this namespace at any given time.
   surface_manager_->UnregisterSurfaceFactoryClient(frame_sink_id_);
-  if (!delegated_local_frame_id_.is_null())
+  if (delegated_local_frame_id_.is_valid())
     factory_.Destroy(delegated_local_frame_id_);
 
   CompositorFrameSink::DetachFromClient();
@@ -88,7 +88,7 @@ void DirectCompositorFrameSink::SubmitCompositorFrame(CompositorFrame frame) {
   gfx::Size frame_size =
       frame.delegated_frame_data->render_pass_list.back()->output_rect.size();
   if (frame_size.IsEmpty() || frame_size != last_swap_frame_size_) {
-    if (!delegated_local_frame_id_.is_null()) {
+    if (delegated_local_frame_id_.is_valid()) {
       factory_.Destroy(delegated_local_frame_id_);
     }
     delegated_local_frame_id_ = surface_id_allocator_.GenerateId();
@@ -105,7 +105,7 @@ void DirectCompositorFrameSink::SubmitCompositorFrame(CompositorFrame frame) {
 }
 
 void DirectCompositorFrameSink::ForceReclaimResources() {
-  if (!delegated_local_frame_id_.is_null()) {
+  if (delegated_local_frame_id_.is_valid()) {
     factory_.SubmitCompositorFrame(delegated_local_frame_id_, CompositorFrame(),
                                    SurfaceFactory::DrawCallback());
   }