Heap::ReportExternalMemoryPressure() uses a slightly different
behavior for incremental marking steps than AdvanceFromTask() or
AdvanceOnAllocation(). This CL switches this method to use
AdvanceOnAllocation().

Bug: v8:12775
Change-Id: If9bb2c2cf3048196286e4b1443129025fbfdf984
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3825780
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82385}