This reverts commit d58bb2dc.

Reason for revert: New test breaks on optimize-for-size:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20-%20debug/builds/16469/steps/OptimizeForSize/logs/GCShortCutting

Original change's description:
> [IdentityMap] Fix size if GC short-cuts objects.
> 
> BUG=chromium:704132
> 
> Change-Id: I6146c907d4f26147676f7dde4974c44fe541e8fe
> Reviewed-on: https://chromium-review.googlesource.com/541362
> Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46059}

TBR=rmcilroy@chromium.org,mstarzinger@chromium.org

Change-Id: Ib2ba207dcc1b3193d3645090e9c0a9676f38c353
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:704132
Reviewed-on: https://chromium-review.googlesource.com/541224Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46061}