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

app/surface/transport_dib_linux.cc

 // Copyright (c) 2009 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.
 
 #include <errno.h>
 #include <stdlib.h>
 #include <sys/ipc.h>
 #include <sys/shm.h>
 
 #include "app/surface/transport_dib.h"
@@ skipping 63 matching lines @@
 }
 
 // static
 TransportDIB* TransportDIB::CreateWithHandle(Handle shmkey) {
   TransportDIB* dib = new TransportDIB;
   dib->key_ = shmkey;
   return dib;
 }
 
 // static
+TransportDIB::Id TransportDIB::InvalidId() {
+  return -1;
+}
+
+// static
 bool TransportDIB::is_valid(Handle dib) {
   return dib >= 0;
 }
 
 skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
   if (address_ == kInvalidAddress && !Map())
     return NULL;
   scoped_ptr<skia::PlatformCanvas> canvas(new skia::PlatformCanvas);
   if (!canvas->initialize(w, h, true, reinterpret_cast<uint8_t*>(memory())))
     return NULL;
@@ skipping 33 matching lines @@
 }
 
 XID TransportDIB::MapToX(Display* display) {
   if (!x_shm_) {
     x_shm_ = ui::AttachSharedMemory(display, key_);
     display_ = display;
   }
 
   return x_shm_;
 }