Request for Review (xs) - 8155966: Assertion failures when -XX:+UseParallelGC -XX:ParallelGCThreads=1

[Jon Masamitsu]

8155966: Assertion failures when -XX:+UseParallelGC -XX:ParallelGCThreads=1
https://bugs.openjdk.java.net/browse/JDK-8155966

The change for 8150994 change the stealing task to the main task for 
compaction.
Mistakenly left in the stealing task definition was code to skip the 
stealing task
if there was only 1 GC thread (no other GC thread from which to steal).

The code changes are an improved assertion message and removal of the code
from the stealing tasks for the special case of 1 GC worker.


http://cr.openjdk.java.net/~jmasa/8155966/webrev.00/

Failure was reproducible with GCOld and ran GCOld with
ParallelGCThreads=1 and default number of GC threads for
testing.

Thanks.

Jon