Avoid accessing |byte_length| during processing buffers. The length
might be a HeapNumber that has already been processed (e.g. moved) in
the current garbage collection cycle.

Bug: v8:8076
Change-Id: I6d79631e300845a29f15a9f60933ee41ffc95300
Reviewed-on: https://chromium-review.googlesource.com/1183193Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55256}