RFR: JDK-8211270: GC abstraction to get real object and headers size

[Roman Kennke]

Shenandoah GC uses one extra word per object. Some places need to know
the real object or header size (fwd ptr plus object).

The affected code paths are WB_GetObjectSize(),
JvmtiEnv::GetObjectSize() and the PLAB constructor. The patch also adds
an assert later in PLAB that uses the header_size().

Bug:
https://bugs.openjdk.java.net/browse/JDK-8211270
Webrev:
file:///home/rkennke/src/openjdk/webrev/JDK-8211270/webrev.00/

Can I please get reviews?

Thanks,
Roman

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