The layout of fixed typed array base is then capable of handling
external typed arrays as well. In a follow-up CL, I'll delete external
typed arrays, and use fixed typed array base instead

BUG=v8:3996
R=jarin@chromium.org,mstarzinger@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1248483007

Cr-Commit-Position: refs/heads/master@{#29786}