RFR: 8202647: Add deduplicate_string function to CollectedHeap

[Stefan Karlsson]

Hi all,

Please review this patch to add a CollectedHeap::deduplicate_string 
virtual function.

https://bugs.openjdk.java.net/browse/JDK-8202647

Today we have this G1 specific code inside the StringTable:

#if INCLUDE_G1GC
  if (G1StringDedup::is_enabled()) {
  // Deduplicate the string before it is interned. Note that we should never
  // deduplicate a string after it has been interned. Doing so will 
counteract
  // compiler optimizations done on e.g. interned string literals.
  G1StringDedup::deduplicate(string());
  }
#endif

This patch adds a new virtual call to CollectedHeap and hides the G1 
specific code inside G1.

I've verified that this doesn't cause any noticeable performance 
regressions with one of Robbin's intern string JMH micro benchmarks 
developed for JDK-8195097.

Thanks,
StefanK