When setting up the initial map for a (class or function) constructor,
we always over-allocate a bunch of in-object properties, in case not
all property assignments happen as `this.prop = val` assignments in
the constructor. However this over-allocation was a bit too aggressive
and added a slack of 8 to each class constructor (plus a minimum of
two, when there was no `this.prop = val` assignment). So in total this
would yield an object with initially 40 in-object property slots in
case of a simple class hierarchy like this:

```js
class A {};
class B extends A {};
class C extends B {};
class D extends C {};
new D;
```

While the slack tracking takes care of eventually shrinking the objects
to appropriate sizes, this aggressive over-allocation is still going to
hurt performance quite a bit in the beginning, and will also lead to
more traffic on the minor GC for now good reason.

Instead of the above, we now allocate a minimum of 2 in-object
properties per class (in a hierarchy) and then add a slack of 8 in the
end. Meaning for the example above we end up with 16 initial in-object
property slots, which seems sensible.

Bug: v8:8853
Change-Id: I4a11e35a8612ceef1d776ca2f0543a26c8c2a2bf
Reviewed-on: https://chromium-review.googlesource.com/c/1477276Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59670}