The logic for V8 GC normally only takes the external memory growth
since last mark-compact into account. Unfortunately, the amount of
external memory recorded at the end of MC is often too high. The
reason is that it might take a while for the external memory
associated with the GCed objects to be released (e.g. V8 itself post a
task to release external memory for ArrayBuffer backing stores). In a
worst case scenario GC is driven only by external memory and none of
the external memory is released by the end of the MC. Then each MC
will record the external memory at its highest point and the GC logic
will allow the external memory to grow a bit higher each time which
can lead to excessive memory use.

This patch improves the situation a bit by calculating the growth from
the lowest external memory seen since the last MC. That way the growth
calculation will be offset from a level presumably closer to the
intended one (to what it would have been if the external memory
associated with the GCed objects was released during the MC). Now,
this fix is not perfect because it can be thrown off by external
memory growth occurring before the lingering memory is
released. However, it seems to work rather well in practice (e.g. when
playing MSE video on YT).

Bug: v8:10185
Change-Id: Ifcdd87eb45f3ae4a99d2aeec667c3ae4ca9a52b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2042711Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66193}