This patch is a workaround to the performance regression caused by
implementing the ES2015 TypedArray prototype chain: Include a
per-TypedArray-subclass length getter so that the superclass getter does
not become polymorphic. The patch appears to fix a regression in the
Gameboy Octane benchmark.

BUG=chromium:579905
R=adamk
LOG=Y

Review URL: https://codereview.chromium.org/1624383003

Cr-Commit-Position: refs/heads/master@{#33501}