Adds a TransportDIB::Id value that is explicitly invalid and use it when compositing

app/surface/transport_dib.h

 // Copyright (c) 2010 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 APP_SURFACE_TRANSPORT_DIB_H_
 #define APP_SURFACE_TRANSPORT_DIB_H_
 #pragma once
 
 #include "base/basictypes.h"
 
@@ skipping 45 matching lines @@
           sequence_num(seq_num) {
     }
 
     bool operator< (const HandleAndSequenceNum& other) const {
       // Use the lexicographic order on the tuple <handle, sequence_num>.
       if (other.handle != handle)
         return other.handle < handle;
       return other.sequence_num < sequence_num;
     }
 
+    bool operator== (const HandleAndSequenceNum& other) const {
+      return other.handle == handle && other.sequence_num == sequence_num;
+    }
+
     HANDLE handle;
     uint32 sequence_num;
   };
   typedef HandleAndSequenceNum Id;
 
   // Returns a default, invalid handle, that is meant to indicate a missing
   // Transport DIB.
   static Handle DefaultHandleValue() { return NULL; }
 
   // Returns a value that is ONLY USEFUL FOR TESTS WHERE IT WON'T BE
@@ skipping 48 matching lines @@
   // Returns NULL on failure.
   static TransportDIB* Map(Handle transport_dib);
 
   // Create a new |TransportDIB| with a handle to the shared memory. This
   // always returns a valid pointer. The DIB is not mapped.
   static TransportDIB* CreateWithHandle(Handle handle);
 
   // Returns true if the handle is valid.
   static bool is_valid(Handle dib);
 
+  // Returns an Id that will never be valid.
+  static Id InvalidId();
+
   // Returns a canvas using the memory of this TransportDIB. The returned
   // pointer will be owned by the caller. The bitmap will be of the given size,
   // which should fit inside this memory.
   //
   // On POSIX, this |TransportDIB| will be mapped if not already. On Windows,
   // this |TransportDIB| will NOT be mapped and should not be mapped prior,
   // because PlatformCanvas will map the file internally.
   //
   // Will return NULL on allocation failure. This could be because the image
   // is too large to map into the current process' address space.
@@ skipping 36 matching lines @@
   void* address_;  // mapped address
   XSharedMemoryId x_shm_;  // X id for the shared segment
   Display* display_;  // connection to the X server
 #endif
   size_t size_;  // length, in bytes
 
   DISALLOW_COPY_AND_ASSIGN(TransportDIB);
 };
 
 #endif  // APP_SURFACE_TRANSPORT_DIB_H_