Instead of using the high water mark for determining this metric, we use
a bitset for all active/used system pages on a V8 heap page. Each time
when allocating a LAB on a page, we add the pages of that memory range
to that bitset. During sweeping we rebuild that bitset from scratch and
replace it with the old one in case free pages are discarded by the GC.
We DCHECK here that the sweeper only ever removes pages. This has the
nice benefit of ensuring that we don't miss any allocations (like we
do now for concurrent allocations).

CommittedPhysicalMemory for a page is then calculated by counting the
set bits in the bitset and multiplying it with the system page size.
This should be simpler to verify and track the "real" effective size
more precisely.

One case where we are partially less precise than the current
implementation is for LABs. In order to reduce complexity we now treat
all pages of a LAB allocation as active immediately. In the current
implementation we tried to only account the actual used part of the LAB
when changing the LAB later. This is more complex to track correctly
but also doesn't account the currently used LAB in effective size.

Change-Id: Ia83df9ad5fbb852f0717c4c396b5074604bd21e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497363Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79428}