Do less work in MultipleProfilers. Reduces runtime from ~8 mins to ~40
seconds.

Bug: v8:8474
Change-Id: I72b3266941ce40c8d064deaf00fb06f8d9fa8a70
Reviewed-on: https://chromium-review.googlesource.com/c/1341956Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57613}