Revert of Avoid number range holes in bitset types. (patchset #5 id:80001 of https://codereview.chromium.org/759013003/)

Reason for revert:
For breaking the waterfall (run-json-stringify test).

Original issue's description:
> Avoid number range holes in bitset types.
>
> BUG=

TBR=rossberg@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/794663002

Cr-Commit-Position: refs/heads/master@{#25756}
Ensure class prototype objects have the right Map::constructor field

The null constructor they had previously could be observed as crashes in
the V8 API's Object::CreationContext() method and in Object.observe.

BUG=v8:3750
LOG=n
R=arv@chromium.org, dslomov@chromium.org

Review URL: https://codereview.chromium.org/787763005

Cr-Commit-Position: refs/heads/master@{#25757}