[mojo-js] Fix uint64 de/serialization

The low 32 bits were being serialized and deserialized as an int32, and
this resulted in 64-bit sign extension when combining the low and high
words on deserialization.

This CL fixes the problem by treating the low 32 bits as a uint32.

Bug: 950745
Change-Id: If9783cfbb7ad8b294bd88f4bcb171a2b91816390
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1565159
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#650892}