Forced GCs are performed during memory benchmarking to record memory usage,
and as such, should not age the bytecode and cause it to be prematurely flushed.

BUG=v8:8395

Change-Id: I75c16d8fec29963e49d70738c6a399fd826f6e8b
Reviewed-on: https://chromium-review.googlesource.com/c/1393298
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58749}