Mojo JS: Use BigInt consistently for 64-bit int fields

This changes 64-bit integer fields to be deserialized as
BigInt values consistently, rather than sometimes being
BigInts and sometimes being Numbers.

Fixed: 1102217,1009262
Change-Id: I9cc8d767058938e9c39f62f139737e512f47cdb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466398
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#818611}