This reverts commit fa314341.

Reason for revert: Causes a significant bug: https://bugs.chromium.org/p/v8/issues/detail?id=6190

Original change's description:
> Stage --harmony-function-tostring
> 
> BUG=v8:4958
> 
> Change-Id: Id02d36fce76eed54a5a3d348dbac2ea7d43f4ef3
> Reviewed-on: https://chromium-review.googlesource.com/462336
> Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
> Commit-Queue: Adam Klein <adamk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44275}

TBR=adamk@chromium.org,littledan@chromium.org,hablich@chromium.org,v8-reviews@googlegroups.com
# Not skipping CQ checks because original CL landed > 1 day ago.
BUG=v8:4958

Change-Id: I43388674e454275fb93a15b9af03e3d8c3cfaaa2
Reviewed-on: https://chromium-review.googlesource.com/465810Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44330}