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..80d83f2f263ddd2b3a112791de598b2839087197 100644
--- a/app/surface/transport_dib_win.cc
+++ b/app/surface/transport_dib_win.cc
@@ -60,6 +60,11 @@ 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.