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

chrome/browser/renderer_host/browser_render_process_host.cc

Index: chrome/browser/renderer_host/browser_render_process_host.cc
diff --git a/chrome/browser/renderer_host/browser_render_process_host.cc b/chrome/browser/renderer_host/browser_render_process_host.cc
index ec7ae7d7500110a97864e5b06ca28da6bdae0f4e..a6f83e918401cc4af958fc7956ef6140b3755dbf 100644
--- a/chrome/browser/renderer_host/browser_render_process_host.cc
+++ b/chrome/browser/renderer_host/browser_render_process_host.cc
@@ -917,12 +917,15 @@ TransportDIB* BrowserRenderProcessHost::MapTransportDIB(
   // for each.
   return widget_helper_->MapTransportDIB(dib_id);
 #elif defined(OS_POSIX)
-  return TransportDIB::Map(dib_id);
+  return TransportDIB::Map(dib_id.shmkey);
 #endif  // defined(OS_POSIX)
 }
 
 TransportDIB* BrowserRenderProcessHost::GetTransportDIB(
     TransportDIB::Id dib_id) {
+  if (!TransportDIB::is_valid_id(dib_id))
+    return NULL;
+
   const std::map<TransportDIB::Id, TransportDIB*>::iterator
       i = cached_dibs_.find(dib_id);
   if (i != cached_dibs_.end()) {