Load the correct 32-bit word to determine if the buffer is shared by
using kBitFieldOffset instead of kBitFieldSlot.

R=ishell@chromium.org,mstarzinger@chromium.org,jkummerow@chromium.org
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2470023003
Cr-Commit-Position: refs/heads/master@{#40730}