Revert of ValueSerializer: Distinguish between 'undefined' and an absent property. (patchset #2 id:20001 of https://codereview.chromium.org/2660093002/ )

Reason for revert:
Seems to break layout tests:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/13146

https://github.com/v8/v8/wiki/Blink-layout-tests

Original issue's description:
> ValueSerializer: Distinguish between 'undefined' and an absent property.
>
> Dealing with this case requires a wire format change. It is possible that an
> element can be absent even in an array where the dense format was chosen
> (because the array initially had no holes), if the elements are modified while
> they are being serialized. In this case, a new tag for the "hole" is emitted.
>
> The logic to treat undefined in dense arrays as an absent property is restricted
> to versions of the wire format that this tag did not exist.
>
> BUG=chromium:686159,chromium:665820
>
> Review-Url: https://codereview.chromium.org/2660093002
> Cr-Commit-Position: refs/heads/master@{#42784}
> Committed: https://chromium.googlesource.com/v8/v8/+/dc85f4c8338c1c824af4f7ee3274dc9f95d14e49

TBR=jkummerow@chromium.org,jbroman@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:686159,chromium:665820

Review-Url: https://codereview.chromium.org/2667553003
Cr-Commit-Position: refs/heads/master@{#42788}