Pass a sequence number along with the DIB handle when compositing in software.

ui/surface/transport_dib.h

Index: ui/surface/transport_dib.h
diff --git a/ui/surface/transport_dib.h b/ui/surface/transport_dib.h
index 233cd2e2f95b18ed8cdad57876373d6cc2a02b14..6397c97420a674572557e7a035ee692c9f678f64 100644
--- a/ui/surface/transport_dib.h
+++ b/ui/surface/transport_dib.h
@@ -54,6 +54,10 @@ class SURFACE_EXPORT TransportDIB {
           sequence_num(seq_num) {
     }
 
+    bool operator==(const HandleAndSequenceNum& other) const {
+      return other.handle == handle && other.sequence_num == sequence_num;
+    }
+
     bool operator<(const HandleAndSequenceNum& other) const {
       // Use the lexicographic order on the tuple <handle, sequence_num>.
       if (other.handle != handle)