RFR: Use current time instead of DBL_MAX for immediate uncommit

[Aleksey Shipilev]

Found this during backports: DBL_MAX is not always trivially accessible. Let's just use current time
for "before" argument, which does the same: uncommitting all regions that were emptied before
current time.

diff -r 3c0dbf1ed0fd src/hotspot/share/gc/shenandoah/shenandoahConcurrentThread.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahConcurrentThread.cpp	Fri Mar 02 22:12:34 2018 +0100
+++ b/src/hotspot/share/gc/shenandoah/shenandoahConcurrentThread.cpp	Mon Mar 05 11:07:09 2018 +0100
@@ -169,7 +169,7 @@
         notify_explicit_gc_waiters();

         // Explicit GC tries to uncommit everything
-        heap->handle_heap_shrinkage(DBL_MAX);
+        heap->handle_heap_shrinkage(os::elapsedTime());
       }

       // If this was the allocation failure GC cycle, notify waiters about it

Testing: hotspot_fast_gc_shenandoah, RSS tests

Thanks,
-Aleksey