In d8 a script is executed in a RealmScope. However, all micro task
which are created by the script are not executed within the RealmScope
at the moment. With this CL I move the execution of the micro task into
the RealmScope.

I thought about creating a new RealmScope for the micro tasks, but
(1) It did not fix the crashing repro;
(2) It seems wrong that the micro tasks are executed in a different
    realm than the script;

Therefore I just moved the execution of the micro tasks into the
RealmScope of the script.

Thereby I moved the execution tasks also into the Context::Scope of the
script. The problem is that the Context::Scope surrounds the RealmScope,
and when I to open the RealmScope before the Context::Scope, not even
the execution of the script works anymore.

R=yangguo@chromium.org

Bug: chromium:797846
Change-Id: If152af282beec8f0b0564dcc9682fee8588e142c
Reviewed-on: https://chromium-review.googlesource.com/856497Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50442}