Revert "MessageInTransit: Fill padding bytes with zero."

This reverts commit ded0aaeec5c8739f18f6c2b0e7d7d248296cdde2.

We're seeing the following on the Windows bot (twice, since your change
landed):

  [ RUN      ] ViewManagerTest.EmbedWithSameViewId2

  [1125/152116:FATAL:window_impl.cc(142)] Check failed: atom. 1410

(I don't believe it's your change, but we'll see. I'll un-revert if it's
still red after the revert.)

TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/754363006