[Translate] Provide a better default value for proto field

Bug: 678689
Change-Id: I9f47f8f59f86621327d5ace0b2431b1fc48ff900
Reviewed-on: https://chromium-review.googlesource.com/889823Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532591}

third_party/metrics_proto/translate_event.proto

@@ -63,7 +63,7 @@ message TranslateEventProto {
     DONT_SHOW = 1;
     NOT_QUERIED = 2;
   }
-  optional RankerResponse ranker_response = 9;
+  optional RankerResponse ranker_response = 9 [default = NOT_QUERIED];
 
   // The action performed by the user in the translate UI.
   // Next tag: 28