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

cc/layers/surface_layer.cc

Index: cc/layers/surface_layer.cc
diff --git a/cc/layers/surface_layer.cc b/cc/layers/surface_layer.cc
index 984a56e684a02a41eb81d9ba55c193222bd10560..815c6471ebca3d40791dcea21ac0dbba15e81cb1 100644
--- a/cc/layers/surface_layer.cc
+++ b/cc/layers/surface_layer.cc
@@ -57,7 +57,7 @@ SurfaceLayer::SurfaceLayer(const SatisfyCallback& satisfy_callback,
 
 SurfaceLayer::~SurfaceLayer() {
   DCHECK(!layer_tree_host());
-  DCHECK(destroy_sequence_.is_null());
+  DCHECK(!destroy_sequence_.is_valid());
 }
 
 void SurfaceLayer::SetSurfaceId(const SurfaceId& surface_id,
@@ -79,7 +79,7 @@ std::unique_ptr<LayerImpl> SurfaceLayer::CreateLayerImpl(
 }
 
 bool SurfaceLayer::HasDrawableContent() const {
-  return !surface_id_.is_null() && Layer::HasDrawableContent();
+  return surface_id_.is_valid() && Layer::HasDrawableContent();
 }
 
 void SurfaceLayer::SetLayerTreeHost(LayerTreeHost* host) {
@@ -104,7 +104,7 @@ void SurfaceLayer::PushPropertiesTo(LayerImpl* layer) {
 }
 
 void SurfaceLayer::CreateNewDestroySequence() {
-  DCHECK(destroy_sequence_.is_null());
+  DCHECK(!destroy_sequence_.is_valid());
   if (layer_tree_host()) {
     destroy_sequence_ = layer_tree_host()
                             ->GetSurfaceSequenceGenerator()
@@ -116,7 +116,8 @@ void SurfaceLayer::CreateNewDestroySequence() {
 void SurfaceLayer::SatisfyDestroySequence() {
   if (!layer_tree_host())
     return;
-  DCHECK(!destroy_sequence_.is_null());
+  // Sometime frame_sink_id_ is null

[piman, 2016/11/08 23:16:34]

What does this comment refer to? There's no frame_sink_id_ here. Is it in
reference to destroy_sequence_.frame_sink_id?

With your change, doesn't that mean the DCHECK is incorrect?

[Alex Z., 2016/11/09 15:29:15]

Sorry about the confusion.
That was one of the notes I left while debugging. It must have somehow escaped
the sweep when I removed my debug statements.
I will remove it in the next patchset.

+  DCHECK(destroy_sequence_.is_valid());
   std::unique_ptr<SatisfySwapPromise> satisfy(
       new SatisfySwapPromise(destroy_sequence_, satisfy_callback_));
   layer_tree_host()->GetSwapPromiseManager()->QueueSwapPromise(