Log group public key hash as a string.

It was previously being logged as an integer, but the value is a long so
it wouldn't obviously correspond to the actual value.

Change-Id: I79fde8d7f86cf929af36d400e2d7df84e1136b0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2150296
Auto-Submit: Max Li <themaxli@chromium.org>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Reviewed-by: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759258}

chromeos/services/device_sync/proto/cryptauth_logging.cc

@@ -231,7 +231,8 @@ base::Value EncryptedGroupPrivateKeyToReadableDictionary(
       (key.encrypted_private_key().empty()
            ? "[Empty]"
            : TruncateStringForLogs(Encode(key.encrypted_private_key()))));
-  dict.SetIntKey("Group public key hash", key.group_public_key_hash());
+  dict.SetStringKey("Group public key hash",
+                    base::NumberToString(key.group_public_key_hash()));
   return dict;
 }