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Adding support for the optional_field_style=reftypes flag.
This is needed for the invalidation client which expects
"boxed" types (e.g., Boolean rather than boolean) for
primitive fields, and depends on null comparisons to
determine whether or not a field is set.

BUG=377891,411282
R=nyquist@chromium.org

Review URL: https://codereview.chromium.org/549543002

Patch from Colin Meek <meek@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#293583}