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}