Retain maps for several garbage collections

This keeps dying maps alive for FLAG_retain_maps_for_n_gc garbage collections to increase chances of them being reused for new objects in future.

BUG=v8:3664
LOG=N
TEST=cctest/test-heap/MapRetaining

Review URL: https://codereview.chromium.org/794583003

Cr-Commit-Position: refs/heads/master@{#25887}
Revert of [turbofan] simplify gap ordering (patchset #2 id:20001 of https://codereview.chromium.org/810013002/)

Reason for revert:
Revert for breaking emscripten bullet with turbofan on android arm64.

Original issue's description:
> [turbofan] simplify gap ordering
>
> BUG=
>
> Committed: https://crrev.com/70b5eb47b39acbf31746f4a116a9b3ce2730218a
> Cr-Commit-Position: refs/heads/master@{#25865}

TBR=bmeurer@chromium.org,dcarney@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/815743002

Cr-Commit-Position: refs/heads/master@{#25888}
Phantom references support internal fields

BUG=

Review URL: https://codereview.chromium.org/753553002

Cr-Commit-Position: refs/heads/master@{#25889}
Reland Shrink initial old generation size based on new space survival rate.

I picked a more conservative start value this time of half max old generation size.

BUG=

Review URL: https://codereview.chromium.org/804323004

Cr-Commit-Position: refs/heads/master@{#25890}