RFR(s): 8038461: Test gc/g1/TestStringDeduplicationMemoryUsage.java fails with unexpected memory usage

[Per Liden]

Hi,

Could I please have this test fix reviewed.

Summary: The test in question tries to calculate the amount of memory 
saved when using string deduplication. The problem is that the JVM doing 
this calculation could be running without compressed class pointers when 
the JVM doing the actual string deduplication is running with compressed 
class pointers. When this happens the JVMs will have different views on 
the size of an array header, which in the end makes the calculation 
incorrect and the test thinks the deduplication failed. Exactly this 
situation happened in a nightly test run. This fix makes sure that the 
array header size of the JVM doing the deduplication is communicated to 
the JVM doing the memory savings calculation.

Bug: https://bugs.openjdk.java.net/browse/JDK-8038461
Webrev: http://cr.openjdk.java.net/~pliden/8038461/webrev.0/

/Per