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This can be useful if a bit of data serialized at the end is optional;
instead of having to encode an explicit boolean flag, the deserializer
can simply consult ReachedEnd().

Bug: 1055911
Change-Id: Id92bfa5ffbdd0c309367417ed12444eff2d94cb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091715
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747909}