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

app/surface/transport_dib.h

Index: app/surface/transport_dib.h
diff --git a/app/surface/transport_dib.h b/app/surface/transport_dib.h
index a55f624c54d08f6172155d89cd177cc360f0be02..8ad4f59c5d32f7c490745c3be725c85c8c703f8f 100644
--- a/app/surface/transport_dib.h
+++ b/app/surface/transport_dib.h
@@ -63,6 +63,10 @@ class TransportDIB {
       return other.sequence_num < sequence_num;
     }
 
+    bool operator== (const HandleAndSequenceNum& other) const {
+      return other.handle == handle && other.sequence_num == sequence_num;
+    }
+
     HANDLE handle;
     uint32 sequence_num;
   };
@@ -131,6 +135,9 @@ class TransportDIB {
   // Returns true if the handle is valid.
   static bool is_valid(Handle dib);
 
+  // Returns an Id that will never be valid.
+  static Id InvalidId();
+
   // Returns a canvas using the memory of this TransportDIB. The returned
   // pointer will be owned by the caller. The bitmap will be of the given size,
   // which should fit inside this memory.