The manual scope is usually used to test corner cases of GC behavior so
disable the ineffective GC detecton there.

Bug: v8:7859
Change-Id: I92a1e43db739846ff0db20a60cc2ace79496e5fe
Reviewed-on: https://chromium-review.googlesource.com/1135140Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54418}