Currently, there is no indication of memory pressure after merging pages
from off-thread space into the heap. This can mean that with lots of
off-thread compilations, we eventually run out of pages in the memory
allocator.

Now, trigger a critical memory pressure notification after merging if
the old generation is close to being unexpandable. We may want to add
other heuristics here (e.g. moderate memory pressure if the heap is
highly fragmented), or possibly add similar checks when the off-thread
space is initialized, but this works for now.

As a drive-by, inline the NotifyOffThreadSpaceMerged call into the
off-thread factory merge -- we already do a bunch of special merging
stuff in there, and nowhere else, so we may as well be explicit in what
we're actually doing. This also allows us to do a pre-check on old-space
size and hard limits just before the merge.

Bug: chromium:1011762
Change-Id: Ic2b46057cf751bd7eaf787f46beaaf4df36e78fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2131915
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67357}