This way we can properly cancel them on the main thread in case they never get
scheduled by the platform.

BUG=chromium:524425
LOG=N
CQ_EXTRA_TRYBOTS=tryserver.v8:v8_linux_arm64_gc_stress_dbg;tryserver.v8:v8_linux_gc_stress_dbg;tryserver.v8:v8_mac_gc_stress_dbg;tryserver.v8:v8_linux64_msan_rel;tryserver.v8:v8_linux64_tsan_rel;tryserver.v8:v8_mac64_asan_rel

Review URL: https://codereview.chromium.org/1446523003

Cr-Commit-Position: refs/heads/master@{#32007}