Revert of [simd.js] Disable SIMD polyfill. (patchset #2 id:20001 of https://codereview.chromium.org/1305923005/ )

Reason for revert:
Breaks simdjs performance tests. See:
https://paste.googleplex.com/5883927122149376

Original issue's description:
> [simd.js] Disable SIMD polyfill.
>
> Disable the polyfill in simd.js tests as the functions for Phase 1 have been implemented.
>
> BUG=v8:4124
> LOG=N
>
> R=bbudge@chromium.org, bmeurer@chromium.org, littledan@chromium.org
>
> Committed: https://crrev.com/749ba3a13558cb018a2ec783e5dfb56d2f8528d8
> Cr-Commit-Position: refs/heads/master@{#30452}

TBR=littledan@chromium.org,bmeurer@chromium.org,bradnelson@chromium.org,bbudge@chromium.org,gdeepti@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4124

Review URL: https://codereview.chromium.org/1308003003

Cr-Commit-Position: refs/heads/master@{#30470}