Fix compile

Author: schmidt-sebastian

firestore/src/main/field_value_main.cc

@@ -294,7 +294,12 @@ std::string Describe(Type type) {
 
 // Helpers
 
-SharedMessage<google_firestore_v1_Value>& GetProtoValue() {
+const SharedMessage<google_firestore_v1_Value>&
+FieldValueInternal::GetProtoValue() const {
+  return absl::get<SharedMessage<google_firestore_v1_Value>>(value_);
+}
+
+SharedMessage<google_firestore_v1_Value>& FieldValueInternal::GetProtoValue() {
   return absl::get<SharedMessage<google_firestore_v1_Value>>(value_);
 }
 

firestore/src/main/field_value_main.h

@@ -74,6 +74,9 @@ class FieldValueInternal {
   explicit FieldValueInternal(FieldValue::Type type, T value)
       : type_{type}, value_{std::move(value)} {}
 
+  /** Returns the underlying value as a google_firestore_v1_Value proto. */
+  const nanopb::SharedMessage<google_firestore_v1_Value>& GetProtoValue() const;
+
   /** Returns the underlying value as a google_firestore_v1_Value proto. */
   nanopb::SharedMessage<google_firestore_v1_Value>& GetProtoValue();