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There are two problems with this API:

1. The buffer size returned is incorrect and could trivially
lead to callers overflowing the message buffer.

2. It does not correctly copy the entire contents of the old message
buffer upon reallocation, instead only copying the extent of the buffer
reported as payload so far.

This fixes both problems. Note that this API has not yet been used in
production, so there are no actual bugs caused by either of these
issues yet.

BUG=742369
R=jcivelli@chromium.org

Change-Id: I7ee7cd0783d641940e0ccafc0c9dc772eeaf2793
Reviewed-on: https://chromium-review.googlesource.com/580252Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488769}