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 42 matching lines @@
 // static
 TransportDIB* TransportDIB::CreateWithHandle(Handle handle) {
   return new TransportDIB(handle);
 }
 
 // static
 bool TransportDIB::is_valid(Handle dib) {
   return dib != NULL;
 }
 
+// static
+TransportDIB::Id TransportDIB::InvalidId() {
+  return Id(DefaultHandleValue(), 0);

[brettw, 2011/03/17 03:31:00]

It seems like this should just use the default constructor so we always have the
default object == InvalidId. We should change the default constructor if it's
not appropriate.

+}
+
 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();
 }
@@ skipping 23 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_);
 }