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

app/surface/transport_dib_mac.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 "app/surface/transport_dib.h"
 
 #include <unistd.h>
 #include <sys/stat.h>
 
 #include "base/eintr_wrapper.h"
@@ skipping 33 matching lines @@
     return NULL;
   return dib.release();
 }
 
 // static
 TransportDIB* TransportDIB::CreateWithHandle(Handle handle) {
   return new TransportDIB(handle);
 }
 
 // static
+TransportDIB::Id TransportDIB::InvalidId() {
+  return -1;
+}
+
+// static
 bool TransportDIB::is_valid(Handle dib) {
   return dib.fd >= 0;
 }
 
 skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
   if (!memory() && !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 20 matching lines @@
   return shared_memory_.memory();
 }
 
 TransportDIB::Id TransportDIB::id() const {
   return shared_memory_.id();
 }
 
 TransportDIB::Handle TransportDIB::handle() const {
   return shared_memory_.handle();
 }