Revert "Revert "MessageInTransit: Fill padding bytes with zero.""

This reverts commit 496472f81d860afecd9e2565f847854baf978ea5.

(It unreverts ded0aaeec5c8739f18f6c2b0e7d7d248296cdde2. Not
surprisingly, the revert didn't fix anything.)

TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/760593007