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Fixes mojom JS lite serialization in cases where fields are
intentionally packed out of order.

This actually simplifies the serialization and deserialization logic
significantly by leveraging precomputed byte/bit offset values from
the bindings generator and preallocating the entire message buffer
prior to serialization.

It also is now correct instead of incorrect, which is nice.

Bug: 904861
Change-Id: If08c301303004d164f4a0afcc3085dac86ca3089
Reviewed-on: https://chromium-review.googlesource.com/c/1335269Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#607980}