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

app/surface/transport_dib_win.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 <windows.h>
 
 #include <limits>
 
@@ skipping 38 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 != NULL;
 }
 
+// static
+bool TransportDIB::is_valid_id(TransportDIB::Id id) {
+  return is_valid_handle(id.handle);
+}
+
 skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
   // This DIB already mapped the file into this process, but PlatformCanvas
   // will map it again.
   DCHECK(!memory()) << "Mapped file twice in the same process.";
 
   scoped_ptr<skia::PlatformCanvas> canvas(new skia::PlatformCanvas);
   if (!canvas->initialize(w, h, true, handle()))
     return NULL;
   return canvas.release();
 }
 
 bool TransportDIB::Map() {
-  if (!is_valid(handle()))
+  if (!is_valid_handle(handle()))
     return false;
   if (memory())
     return true;
 
   if (!shared_memory_.Map(0 /* map whole shared memory segment */)) {
     LOG(ERROR) << "Failed to map transport DIB"
                << " handle:" << shared_memory_.handle()
                << " error:" << ::GetLastError();
     return false;
   }
@@ skipping 10 matching lines @@
   return shared_memory_.memory();
 }
 
 TransportDIB::Handle TransportDIB::handle() const {
   return shared_memory_.handle();
 }
 
 TransportDIB::Id TransportDIB::id() const {
   return Id(handle(), sequence_num_);
 }