Revert "Remove the notion of “pending” rules."

We got enough reports of memory regressions that we need to put back
into place the old regime; it's not immediately clear that all of them
are truly bad (e.g. maximum RAM used might be less important, if we
deallocate most of it afterwards), but the sum seems risky enough that
the disadvantages outweigh the benefits.

This reverts commit 41e65a4b.

R=andruud@chromium.org

Fixed: 1325683
Fixed: 1325700
Fixed: 1325722
Change-Id: Ib2c50a5841103ba7b313f48072ef402182252a03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3654213


Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1005205}