On the PredictablePlatform, worker tasks were executed immediately
instead of posting them in a task queue first. This approach caused
problems because the execution of the worker task blocked progress of
the posting task, and the worker task was always executed in the
context of the posting task, e.g. with an already open HandleScope.

With this CL, worker tasks get posted into the foreground task queue
of the nullptr isolate instead of executing them immediately.
The tasks of the nullptr isolate are then executed after a task of
some other task queue is executed. As the worker tasks are thereby
executed on the same thread as foreground tasks, the behavior is
deterministic.

A consequence of this approach is that each pumping the message loop
of an Isolate may also execute other Isolate's background tasks.

This approach is needed because we don't have a BackgroundTaskRunner but
merely a CallOnWorkerThread method that doesn't know which Isolate the
task corresponds to.

R=clemensb@chromium.org, mlippautz@chromium.org

Bug: v8:9670
Change-Id: I6847ae042146431bc2376d27280be8829f529b95
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2182453
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67616}

Bug: v8:9247
Change-Id: I404e34a013b95c6a7cf03dd9ec0732d9ad796bdc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1613238
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61531}

This flag stresses different interleavings of background and foreground
tasks by delaying the execution of each task by a random value between
0 and 100ms (with a quadratic distribution favoring smaller delayes).

The implementation is encapsulated in the new {DelayedTasksPlatform}
class, which wraps each task in a {DelayedTask} which first sleeps for
the given number of microseconds, then executes the actual task.

Both the old {PredictablePlatform} and the new {DelayedTasksPlatform}
are moved to the new d8-platforms.cc file with an interface to create
them in d8-platforms.h.

R=yangguo@chromium.org, mslekova@chromium.org

Bug: v8:8278
Change-Id: I5847fb2da31ffde773195da7ad3f56a0390cc05b
Reviewed-on: https://chromium-review.googlesource.com/c/1270592
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56496}