RFR: 8183925: Decouple crash protection from watcher thread

[Robbin Ehn]

Hi all, please review:

While looking into:
java_lang_Thread::get_thread_status(oop java_thread) {
      4802:   assert(Thread::current()->is_Watcher_thread()
As: http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2017-July/023863.html

I saw that I have missed that crash protection doesn't work for the upcoming trace backend changes.
Main reason for not seeing this is that the trace backend haven't crashed (yet).

This code is not used in the open-jdk, compiled tested linux and windows.

Bug: https://bugs.openjdk.java.net/browse/JDK-8183925
Webrev: http://cr.openjdk.java.net/~rehn/8183925/webrev/index.html

Thanks!

/Robbin