It was used before as a placeholder in Result<DecodeStruct*> to
communicate that no value was returned. We actually only created a
Results holding {nullptr} when returning such values. Thus, the whole
struct is not needed, and we return Result<nullptr_t> instead, which
clearly communicates that this result does not hold any value.

An alternative would be to use Result<void>, but this would require
partial specialization of the Result template, which would be overkill
here.

R=ahaas@chromium.org

Change-Id: Ib07d2c4fe716c735839675d11146c47f97997d40
Reviewed-on: https://chromium-review.googlesource.com/509551Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45464}