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

app/surface/transport_dib_win.cc

Index: app/surface/transport_dib_win.cc
diff --git a/app/surface/transport_dib_win.cc b/app/surface/transport_dib_win.cc
index 22812c9d8400a29448209e5cd4981eaff4b09713..3de7fab4dad0a51ca1c03153115e89ae07a1bed3 100644
--- a/app/surface/transport_dib_win.cc
+++ b/app/surface/transport_dib_win.cc
@@ -56,10 +56,15 @@ TransportDIB* TransportDIB::CreateWithHandle(Handle 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.
@@ -72,7 +77,7 @@ skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
 }
 
 bool TransportDIB::Map() {
-  if (!is_valid(handle()))
+  if (!is_valid_handle(handle()))
     return false;
   if (memory())
     return true;