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/1114539Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54022}