[Autofill Assistant] Changed feedback error message to address potential security issue.

Bug: 806868
Change-Id: Iee8f1ff42f8c76b7553ffce55d51ca5e54afd95b
Reviewed-on: https://chromium-review.googlesource.com/c/1340818Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#609233}

chrome/android/java/src/org/chromium/chrome/browser/autofill_assistant/FeedbackContext.java

@@ -20,7 +20,9 @@ class FeedbackContext extends JSONObject {
             return new FeedbackContext(activity, client, details, statusMessage)
                     .toString(indentSpaces);
         } catch (JSONException e) {
-            return "{\"error\": \"" + e.getMessage() + "\"}";
+            // Note: it is potentially unsafe to return e.getMessage(): the exception message
+            // could be wrangled and used as an attack vector when arriving at the JSON parser.
+            return "{\"error\": \"Failed to convert feedback context to string.\"}";
         }
     }