Build fix

Author: schmidt-sebastian

firebase-firestore/src/test/java/com/google/firebase/firestore/model/mutation/MutationTest.java

@@ -564,7 +564,7 @@ private void assertVersionTransitions(
       MutableDocument base,
       MutationResult mutationResult,
       MutableDocument expected) {
-    MutableDocument clone = base.clone();
+    MutableDocument clone = base.mutableCopy();
     mutation.applyToRemoteDocument(clone, mutationResult);
     assertEquals(expected, clone);
   }
@@ -1033,8 +1033,8 @@ private String getDescription(
   }
 
   private void verifyOverlayRoundTrips(MutableDocument doc, Mutation... mutations) {
-    MutableDocument docForMutations = doc.clone();
-    MutableDocument docForOverlay = doc.clone();
+    MutableDocument docForMutations = doc.mutableCopy();
+    MutableDocument docForOverlay = doc.mutableCopy();
     Timestamp now = Timestamp.now();
 
     FieldMask mask = null;