RFR: 8286737: Test vmTestbase/gc/gctests/WeakReference/weak006/weak006.java fails: Last soft reference has not been cleared

[Leonid Mesnik]

The problem is that PSScavenge::invoke()  (young gen GC) never tries to clear soft references. 

The fix is to update the clear_all_soft_refs to be the same as in full GC if scavenge is run as a part of full GC.  Test passed before https://bugs.openjdk.java.net/browse/JDK-8285951 Replace Algorithms.eatMemory(...) with WB.fullGC() in vmTestbase_vm_gc_ref tests because the eatMemory causes a lot of full GC and all references were propagated into old gen.  

The failed mem allocation also runs full GC with clear_all_soft_refs = true before throwing OOME so it should be also ok.

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Commit messages:
 - 8286737

Changes: https://git.openjdk.java.net/jdk/pull/8829/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8829&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286737
  Stats: 58 lines in 5 files changed: 17 ins; 30 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8829.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8829/head:pull/8829

PR: https://git.openjdk.java.net/jdk/pull/8829