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

test/cctest/test-heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 5533 matching lines @@
   Isolate* isolate = CcTest::i_isolate();
   Heap* heap = isolate->heap();
   GCTracer* tracer = heap->tracer();
   int time1 = 100;
   size_t counter1 = 1000;
   tracer->SampleNewSpaceAllocation(time1, counter1);
   int time2 = 200;
   size_t counter2 = 2000;
   tracer->SampleNewSpaceAllocation(time2, counter2);
   size_t bytes = tracer->NewSpaceAllocatedBytesInLast(1000);
-  CHECK_EQ(0, bytes);
+  CHECK_EQ(10000, bytes);
   int time3 = 1000;
   size_t counter3 = 30000;
   tracer->SampleNewSpaceAllocation(time3, counter3);
   bytes = tracer->NewSpaceAllocatedBytesInLast(100);
   CHECK_EQ((counter3 - counter1) * 100 / (time3 - time1), bytes);
 }