A few coverage tests depend on a GC run that deterministically frees all
dead objects, but Runtime::kCollectGarbage did not explicitly disable
incremental marking. Incremental marking makes liveness detection
timing-dependent and thus should be disabled here.

NOTRY=true
NOPRESUBMIT=true
TBR=mlippautz@chromium.org

Bug: v8:7108
Change-Id: I8bebe612bbc2126b8ad778af15f08442ccc91a35
Reviewed-on: https://chromium-review.googlesource.com/781865
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49544}