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

chrome/browser/renderer_host/browser_render_process_host.cc

 // Copyright (c) 2011 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.
 
 // Represents the browser side of the browser <--> renderer communication
 // channel. There will be one RenderProcessHost per renderer process.
 
 #include "chrome/browser/renderer_host/browser_render_process_host.h"
 
 #include <algorithm>
@@ skipping 904 matching lines @@
   // On OSX, the browser allocates all DIBs and keeps a file descriptor around
   // for each.
   return widget_helper_->MapTransportDIB(dib_id);
 #elif defined(OS_POSIX)
   return TransportDIB::Map(dib_id);
 #endif  // defined(OS_POSIX)
 }
 
 TransportDIB* BrowserRenderProcessHost::GetTransportDIB(
     TransportDIB::Id dib_id) {
+  if (dib_id == TransportDIB::InvalidId())

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

We should be sure to comment the valid cases where this may happen.

+    return NULL;
+
   const std::map<TransportDIB::Id, TransportDIB*>::iterator
       i = cached_dibs_.find(dib_id);
   if (i != cached_dibs_.end()) {
     cached_dibs_cleaner_.Reset();
     return i->second;
   }
 
   TransportDIB* dib = MapTransportDIB(dib_id);
   if (!dib)
     return NULL;
@@ skipping 365 matching lines @@
     IPC::PlatformFileForTransit file;
 #if defined(OS_POSIX)
     file = base::FileDescriptor(model_file, false);
 #elif defined(OS_WIN)
     ::DuplicateHandle(::GetCurrentProcess(), model_file, GetHandle(), &file, 0,
                       false, DUPLICATE_SAME_ACCESS);
 #endif
     Send(new ViewMsg_SetPhishingModel(file));
   }
 }