Add memory pressure notification API

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 44a080b1e0914ddbd9c40772bcd7d2cc03c3b393..aaa9b7c5228ee84bcad679379e1c141944451d95 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -412,6 +412,8 @@ enum ArrayStorageAllocationMode {
 
 enum class ClearRecordedSlots { kYes, kNo };
 
+enum class MemoryPressureLevel { kNone, kModerate, kCritical };

[jochen (gone - plz use gerrit), 2016/03/17 19:33:39]

why not just use the external class?

[ulan, 2016/03/18 12:49:41]

Done.

+
 class Heap {
  public:
   // Declare all the root indices.  This defines the root list order.
@@ -736,6 +738,9 @@ class Heap {
   bool IdleNotification(double deadline_in_seconds);
   bool IdleNotification(int idle_time_in_ms);
 
+  void MemoryPressureNotification(MemoryPressureLevel level,
+                                  bool is_isolate_locked);
+
   double MonotonicallyIncreasingTimeInMs();
 
   void RecordStats(HeapStats* stats, bool take_snapshot = false);
@@ -833,7 +838,10 @@ class Heap {
 
   void SetOptimizeForLatency() { optimize_for_memory_usage_ = false; }
   void SetOptimizeForMemoryUsage();
-  bool ShouldOptimizeForMemoryUsage() { return optimize_for_memory_usage_; }
+  bool ShouldOptimizeForMemoryUsage() {
+    return optimize_for_memory_usage_ ||
+           memory_pressure_level_.Value() != MemoryPressureLevel::kNone;
+  }
 
   // ===========================================================================
   // Initialization. ===========================================================
@@ -1996,6 +2004,8 @@ class Heap {
   // count, as scopes can be acquired from multiple tasks (read: threads).
   AtomicNumber<size_t> always_allocate_scope_count_;
 
+  AtomicValue<MemoryPressureLevel> memory_pressure_level_;
+
   // For keeping track of context disposals.
   int contexts_disposed_;