[8u] RFR (XS) 8134030: test/serviceability/dcmd/gc/HeapDumpTest fails to verify the dump

[Aleksey Shipilev]

Original fix:
  https://bugs.openjdk.java.net/browse/JDK-8134030
  http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/2a591a32d924

There is nowhere to put the test in 8u, and the shape of affected code is a bit different. However,
the patch itself is simple. It can also subsume one of the previous asserts by doing
InstanceKlass::cast.

8u webrev:
  http://cr.openjdk.java.net/~shade/8134030/webrev.8u.01/

Testing: Linux x86_64 fastdebug; all hotspot jtregs

-- 
Thanks,
-Aleksey