This reverts commit f0bcbc90.

Reason for revert: Still failing bots.

Original change's description:
> Reland "Introduce MutableHeapNumber class."
> 
> This is a reland of 40ac6b18, which
> was incorrect due to a bad merge.
> 
> 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=bmeurer@chromium.org
> TBR=ulanchromium.org
> 
> Change-Id: I3af1014c949821dfac0754a3e48c65ce1bad1ad1
> Reviewed-on: https://chromium-review.googlesource.com/1114539
> Reviewed-by: Georg Neis <neis@chromium.org>
> Commit-Queue: Georg Neis <neis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54022}

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

Change-Id: I99c226e95dfb0b913903cc83193f6e51de8c1b47
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1114099Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54024}