Activate memory reducer for small heaps in background tabs.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index d4fbbc70b284ea2837822fa7669ae483c5002bdf..01e84f8315efcfd67f676e075fc45c60a42c41e8 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -1060,9 +1060,9 @@ int Heap::NotifyContextDisposed(bool dependant_context) {
     tracer()->ResetSurvivalEvents();
     old_generation_size_configured_ = false;
     MemoryReducer::Event event;
-    event.type = MemoryReducer::kContextDisposed;
+    event.type = MemoryReducer::kPossibleGarbage;
     event.time_ms = MonotonicallyIncreasingTimeInMs();
-    memory_reducer_->NotifyContextDisposed(event);
+    memory_reducer_->NotifyPossibleGarbage(event);
   }
   if (isolate()->concurrent_recompilation_enabled()) {
     // Flush the queued recompilation tasks.
@@ -4112,6 +4112,17 @@ bool Heap::HasHighFragmentation(intptr_t used, intptr_t committed) {
   return committed - used > used + kSlack;
 }
 
+void Heap::SetOptimizeForMemoryUsage() {
+  // 2 pages for each old generation space + 1 page for new space.

[jochen (gone - plz use gerrit), 2016/02/22 09:40:09]

that's lo_space and old_space and?

[ulan, 2016/02/22 10:05:32]

Expanded the comment. It is old, code, and map space. For small heaps lo space
should be close to 0.

+  const int kMinCommittedMemory = 7 * Page::kPageSize;
+  if (ms_count_ == 0 && CommittedMemory() > kMinCommittedMemory) {

[jochen (gone - plz use gerrit), 2016/02/22 09:40:09]

I guess this method is invoked every time a tab goes to the background?

[ulan, 2016/02/22 10:05:32]

yes

+    MemoryReducer::Event event;
+    event.type = MemoryReducer::kPossibleGarbage;
+    event.time_ms = MonotonicallyIncreasingTimeInMs();
+    memory_reducer_->NotifyPossibleGarbage(event);
+  }
+  optimize_for_memory_usage_ = true;
+}
 
 void Heap::ReduceNewSpaceSize() {
   // TODO(ulan): Unify this constant with the similar constant in