This reverts commit 8196e102.

Reason for revert: Performance regression due to hashcode lookup.

Original change's description:
> [builtins] Move most WeakMap/WeakSet code from JS to C++ builtins
> 
> They were already implemented mostly in C++ (only error/negative
> cases were handled in script), so this is mostly just a cleanup.
> Only the constructors remain in script after this CL.
> 
> Bug: v8:6354
> Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
> Change-Id: I5b3579337a8e33dc30d49c2da5cfd42baec697bb
> Reviewed-on: https://chromium-review.googlesource.com/531670
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
> Commit-Queue: Adam Klein <adamk@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#45924}

TBR=adamk@chromium.org,cbruni@chromium.org,gsathya@chromium.org
Bug: v8:6354, chromium:733238
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng

Change-Id: Ia5a741b9587886298f3ca057f6a6adeba556b8e0
Reviewed-on: https://chromium-review.googlesource.com/537207Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45966}