CRR: 6847956: G1: crash in oopDesc*G1ParCopyHelper::copy_to_survivor_space(oopDesc*)

[Tony Printezis]

The CR has a detailed explanation of why marking can be incomplete when 
survivor spaces are enabled. The fix is straightforward:

http://cr.openjdk.java.net/~tonyp/6847956/webrev.0/

The test case that would fail within 30-40 mins, ran for over 12 hours. 
I'll do more testing just in case, but I thought I'd open this for code 
review to get it ready for push asap.

Tony