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

cc/surfaces/surface_id.h

Index: cc/surfaces/surface_id.h
diff --git a/cc/surfaces/surface_id.h b/cc/surfaces/surface_id.h
index 67477714c98894b5c2e7cad8ec82f686edb2eb92..ae16b232bfbc7c5f1332daf1a8c88e29ac29befb 100644
--- a/cc/surfaces/surface_id.h
+++ b/cc/surfaces/surface_id.h
@@ -40,8 +40,8 @@ class SurfaceId {
                       const LocalFrameId& local_frame_id)
       : frame_sink_id_(frame_sink_id), local_frame_id_(local_frame_id) {}
 
-  bool is_null() const {
-    return frame_sink_id_.is_null() && local_frame_id_.is_null();
+  bool is_valid() const {
+    return frame_sink_id_.is_valid() && local_frame_id_.is_valid();

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

This changes the logic. Is that intended?

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

Yes, it is intended.
We used to assume that a not-null SurfaceId is a valid SurfaceId, which turned
out to be false.
I will add more context to the CL's description.

   }
 
   size_t hash() const {