Update mojo sdk to rev 1dc8a9a5db73d3718d99917fadf31f5fb2ebad4f

third_party/mojo/src/mojo/public/go/bindings/decoder.go

Index: third_party/mojo/src/mojo/public/go/bindings/decoder.go
diff --git a/third_party/mojo/src/mojo/public/go/bindings/decoder.go b/third_party/mojo/src/mojo/public/go/bindings/decoder.go
index 9711e23b4473e8cb9fe4034ab215abd962b34987..dac511021e56387ad9de8ffbfa18d8e278862934 100644
--- a/third_party/mojo/src/mojo/public/go/bindings/decoder.go
+++ b/third_party/mojo/src/mojo/public/go/bindings/decoder.go
@@ -149,6 +149,21 @@ func (d *Decoder) StartStruct() (DataHeader, error) {
 	return header, nil
 }
 
+// StartUnion starts decoding a union and reads its header.
+// Returns the read data header. The caller should check if it is valid.
+// Note: it doesn't read the data field.
+func (d *Decoder) StartUnion() (DataHeader, error) {
+	header, err := d.readDataHeader()
+	if err != nil {
+		return DataHeader{}, err
+	}
+
+	if err := d.pushState(header, 0); err != nil {
+		return DataHeader{}, err
+	}
+	return header, nil
+}
+
 func (d *Decoder) readDataHeader() (DataHeader, error) {
 	if err := d.claimData(dataHeaderSize); err != nil {
 		return DataHeader{}, err