Allow layers to signal that additional sequences are needed before surface destruction

cc/surfaces/surface_sequence.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_SURFACES_SURFACE_SEQUENCE_H_
 #define CC_SURFACES_SURFACE_SEQUENCE_H_
 
 namespace cc {
 
 // A per-surface-namespace sequence number that's used to coordinate
 // dependencies between frames. A sequence number may be satisfied once, and
 // may be depended on once.
 struct SurfaceSequence {
   SurfaceSequence() : id_namespace(0u), sequence(0u) {}
   SurfaceSequence(uint32_t id_namespace, uint32_t sequence)
       : id_namespace(id_namespace), sequence(sequence) {}
+  bool is_null() const { return id_namespace == 0u && sequence == 0u; }
 
   uint32_t id_namespace;
   uint32_t sequence;
 };
 
 inline bool operator==(const SurfaceSequence& a, const SurfaceSequence& b) {
   return a.id_namespace == b.id_namespace && a.sequence == b.sequence;
 }
 
 inline bool operator!=(const SurfaceSequence& a, const SurfaceSequence& b) {
   return !(a == b);
 }
 
 inline bool operator<(const SurfaceSequence& a, const SurfaceSequence& b) {
   if (a.id_namespace != b.id_namespace)
     return a.id_namespace < b.id_namespace;
   return a.sequence < b.sequence;
 }
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_SEQUENCE_H_