RFR: JDK-8204685: Abstraction for TLAB dummy object

[Roman Kennke]

Similar to how object allocations should be owned by the GC, so does
TLAB dummy object 'allocation'. TLABs and PLABs are filling their
remaining blocks with dummy objects. GCs like Shenandoah might need a
slightly differet layout for this, and therefore we need an abstraction.

The proposed change adds a new virtual method
CollectedHeap::fill_with_dummy_object(), the default implementation
calls the existing CH::fill_with_object() from TLAB and PLAB like it was
done before.

Tested: hotspot-tier1, Shenandoah tests with appropriate impl

http://cr.openjdk.java.net/~rkennke/JDK-8204685/webrev.00/

Ok?

Thanks, Roman

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