This reverts commit 40ac6b18.

Reason for revert: https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Linux%20-%20debug/21009

Original change's description:
> Introduce MutableHeapNumber class.
> 
> V8 knows heap numbers and mutable heap numbers. They have
> difference instance types, but in C++ code we've used the
> same class for both (HeapNumber). Confusingly, however,
> IsHeapNumber would return false for mutable heap numbers,
> while HeapNumber::cast would succeed.
> 
> This CL adds a separate class MutableHeapNumber and
> eliminates the confusing behavior.
> 
> TBR=bmeurer@chromium.org
> 
> Change-Id: Id894d177c7fe8cc3f451be80c273b50daee91378
> Reviewed-on: https://chromium-review.googlesource.com/1113544
> Commit-Queue: Georg Neis <neis@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54012}

TBR=ulan@chromium.org,jarin@chromium.org,neis@chromium.org,bmeurer@chromium.org

Change-Id: I358a822f20b9110def968e69463a753a2a32c68c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1114538Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54013}