This CL introduced SafeForInterruptsScope. This scope overrides
outer PostponeInterruptsScopes:
- reschedule postponed interrupts if needed,
- allow requesting new interrupts.
As soon as scope removed interrupts are posponed if needed.

This scope will be:
- used to allow inspector to interrupt and terminate
  DebugeEvaluate::Local,
- exposed with new flag on Isolate to implement SafeForTerminationScope
  in blink.

R=yangguo@chromium.org

Bug: chromium:820640
Change-Id: I15befc10c2cee393d1e3be48cecb31ee14dae638
Reviewed-on: https://chromium-review.googlesource.com/1022969
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52743}