Uncommit and shrink semi-spaces only on low allocation rate.

src/heap/gc-tracer.cc

Index: src/heap/gc-tracer.cc
diff --git a/src/heap/gc-tracer.cc b/src/heap/gc-tracer.cc
index 927915d7554bae15b4e26c43b997734824f8c231..815baa440c308bf4fd619929e74a09db7fa11cf3 100644
--- a/src/heap/gc-tracer.cc
+++ b/src/heap/gc-tracer.cc
@@ -610,7 +610,7 @@ size_t GCTracer::NewSpaceAllocatedBytesInLast(double time_ms) const {
     ++iter;
   }
 
-  if (durations < time_ms) return 0;
+  if (durations == 0.0) return 0;
 
   bytes = static_cast<size_t>(bytes * (time_ms / durations) + 0.5);
   // Return at least 1 since 0 means "no data".
@@ -618,6 +618,15 @@ size_t GCTracer::NewSpaceAllocatedBytesInLast(double time_ms) const {
 }
 
 
+size_t GCTracer::CurrentNewSpaceAllocationThroughputInBytesPerMillisecond()
+    const {
+  static const double kThroughputTimeFrame = 5000;
+  size_t allocated_bytes = NewSpaceAllocatedBytesInLast(kThroughputTimeFrame);
+  if (allocated_bytes == 0) return 0;
+  return static_cast<size_t>((allocated_bytes / kThroughputTimeFrame) + 1);
+}
+
+
 double GCTracer::ContextDisposalRateInMilliseconds() const {
   if (context_disposal_events_.size() < kRingBufferMaxSize) return 0.0;