Fix compile

Author: schmidt-sebastian

firestore/src/main/field_value_main.cc

@@ -294,7 +294,8 @@ 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_);
 }
 

firestore/src/main/field_value_main.h

@@ -75,7 +75,7 @@ class FieldValueInternal {
       : type_{type}, value_{std::move(value)} {}
 
   /** Returns the underlying value as a google_firestore_v1_Value proto. */
-  nanopb::SharedMessage<google_firestore_v1_Value>& GetProtoValue();
+  const nanopb::SharedMessage<google_firestore_v1_Value>& GetProtoValue() const;
 
   FieldValue::Type type_ = FieldValue::Type::kNull;
   // Note: it's impossible to roundtrip between a `DocumentReference` and