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
-bool TransportDIB::is_valid(Handle dib) {
+bool TransportDIB::is_valid_handle(Handle dib) {
   return dib.fd >= 0;
 }
 
+// static
+bool TransportDIB::is_valid_id(Id id) {
+  return id != 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;
   return canvas.release();
 }
 
 bool TransportDIB::Map() {
-  if (!is_valid(handle()))
+  if (!is_valid_handle(handle()))
     return false;
   if (memory())
     return true;
 
   struct stat st;
   if ((fstat(shared_memory_.handle().fd, &st) != 0) ||
       (!shared_memory_.Map(st.st_size))) {
     return false;
   }
 
   size_ = st.st_size;
   return true;
 }
 
 void* TransportDIB::memory() const {
   return shared_memory_.memory();
 }
 
 TransportDIB::Id TransportDIB::id() const {
   return shared_memory_.id();
 }
 
 TransportDIB::Handle TransportDIB::handle() const {
   return shared_memory_.handle();
 }