[objects] do not visit ArrayBuffer's backing store

[objects] do not visit ArrayBuffer's backing store

ArrayBuffer's backing store is a pointer to external heap, and can't be
treated as a heap object. Doing so will result in crashes, when the
backing store is unaligned.

See: https://github.com/nodejs/node/issues/2791

BUG=chromium:530531
R=mlippautz@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1327403002

Cr-Commit-Position: refs/heads/master@{#30771}