Activate memory reducer for small heaps in background tabs.

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 803 matching lines @@
   inline void OnMoveEvent(HeapObject* target, HeapObject* source,
                           int size_in_bytes);
 
   bool deserialization_complete() const { return deserialization_complete_; }
 
   bool HasLowAllocationRate();
   bool HasHighFragmentation();
   bool HasHighFragmentation(intptr_t used, intptr_t committed);
 
   void SetOptimizeForLatency() { optimize_for_memory_usage_ = false; }
-  void SetOptimizeForMemoryUsage() { optimize_for_memory_usage_ = true; }
+  void SetOptimizeForMemoryUsage();
   bool ShouldOptimizeForMemoryUsage() { return optimize_for_memory_usage_; }
 
   // ===========================================================================
   // Initialization. ===========================================================
   // ===========================================================================
 
   // Configure heap size in MB before setup. Return false if the heap has been
   // set up already.
   bool ConfigureHeap(int max_semi_space_size, int max_old_space_size,
                      int max_executable_size, size_t code_range_size);
@@ skipping 1791 matching lines @@
   friend class LargeObjectSpace;
   friend class NewSpace;
   friend class PagedSpace;
   DISALLOW_COPY_AND_ASSIGN(AllocationObserver);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_