Truncate varint to 32-bit integer.
Truncate varint to 32-bit integer.

This reverts an unintentional change in CL 226346424 where on 32-bit platforms
varint was implicitly truncated in some cases but not the others.

gfe-relnote: n/a (only affects 32-bit platforms)
PiperOrigin-RevId: 229679285
Change-Id: I962f50e90fd7273f3348f34240d14f5737c9fd12

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