device/fido: update the ClientDataJson GREASE string.

Good to keep things GREASEd.

Change-Id: I7eaaa3ea9a6eb4afb248574c39f1da17f5dad246
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410929
Auto-Submit: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#806950}

device/fido/client_data.cc

@@ -102,7 +102,7 @@ std::string SerializeCollectedClientDataToJson(
     // unreasonably specific assumptions about the clientData JSON. This is
     // done in the fashion of
     // https://tools.ietf.org/html/draft-ietf-tls-grease
-    ret.append(R"(,"extra_keys_may_be_added_here":")");
+    ret.append(R"(,"other_keys_can_be_added_here":")");
     ret.append(
         "do not compare clientDataJSON against a template. See "
         "https://goo.gl/yabPex\"");