This change binds each {Cancelable} task to a so-called {CancelableTaskManager},
which is then used to handle concurrent cancelation as well as synchronizing
shutdown for already running tasks. Since ownership of tasks is transferred to
the platform executing a task (destructor), handling in the manager uses integer
ids. Note that this also mitigates (modulo integer size) the ABA problem.
All handling of {Cancelable} tasks is now encapsulated into the corresponding
manager, which is instantiated for each isolate.
R=hpayer@chromium.org
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/1409993012
Cr-Commit-Position: refs/heads/master@{#31997}
