This reverts commit 5d8840e2.

Reason for revert: performance regressions.

BUG=chromium:737472,chromium:736640

Original change's description:
> [runtime] Copy descriptors on prototype map transition.
> 
> Currently the descriptors are moved from the old map to the new map,
> which is unsafe for the concurrent marker. This patch removes the map
> mutation.
> 
> Change-Id: I3f7ce455c7344148a122c7443cf32a4eef0307be
> Reviewed-on: https://chromium-review.googlesource.com/535480
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46131}

TBR=ulan@chromium.org,verwaest@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I2525a8101a01f39ccad1a6e49aae1e3f28da5b5d
Reviewed-on: https://chromium-review.googlesource.com/555551Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46745}