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

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_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <tuple>
 
 #include "base/hash.h"
 #include "cc/surfaces/frame_sink_id.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() : sequence(0u) {}
   SurfaceSequence(const FrameSinkId& frame_sink_id, uint32_t sequence)
-      : frame_sink_id(frame_sink_id), sequence(sequence) {}
-  bool is_null() const { return frame_sink_id.is_null() && sequence == 0u; }
+      : frame_sink_id(frame_sink_id), sequence(sequence) {
+    DCHECK(frame_sink_id.is_valid());
+  }
+  bool is_valid() const { return frame_sink_id.is_valid() && sequence > 0u; }
 
   FrameSinkId frame_sink_id;
   uint32_t sequence;
 };
 
 inline bool operator==(const SurfaceSequence& a, const SurfaceSequence& b) {
   return a.frame_sink_id == b.frame_sink_id && 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) {
   return std::tie(a.frame_sink_id, a.sequence) <
          std::tie(b.frame_sink_id, b.sequence);
 }
 
 struct SurfaceSequenceHash {
   size_t operator()(SurfaceSequence key) const {
     return base::HashInts(static_cast<uint64_t>(key.frame_sink_id.hash()),
                           key.sequence);
   }
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_SEQUENCE_H_