This is the first step towards unifying external and on-heap typed arrays.

The end-state will be that this base pointer either points to the on-heap values
or to the externally allocated array buffer.

BUG=v8:3996
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1176263004

Cr-Commit-Position: refs/heads/master@{#29039}