Initial idle GC logic.

runtime/vm/scavenger.h

Index: runtime/vm/scavenger.h
diff --git a/runtime/vm/scavenger.h b/runtime/vm/scavenger.h
index 4d59fc862e50354d70ae9d49d539f4f06829014d..53d0dd32de7b22fae345d2e67ea81c83c9b55f6e 100644
--- a/runtime/vm/scavenger.h
+++ b/runtime/vm/scavenger.h
@@ -97,6 +97,8 @@ class ScavengeStats {
                : 0.0;
   }
 
+  intptr_t UsedBeforeInWords() const { return before_.used_in_words; }
+
   int64_t DurationMicros() const { return end_micros_ - start_micros_; }
 
  private:
@@ -201,6 +203,8 @@ class Scavenger {
 
   void WriteProtect(bool read_only);
 
+  bool ShouldPerformIdleScavenge(int64_t deadline);
+
   void AddGCTime(int64_t micros) { gc_time_micros_ += micros; }
 
   int64_t gc_time_micros() const { return gc_time_micros_; }
@@ -307,9 +311,12 @@ class Scavenger {
 
   int64_t gc_time_micros_;
   intptr_t collections_;
-  static const int kStatsHistoryCapacity = 2;
+  static const int kStatsHistoryCapacity = 4;
   RingBuffer<ScavengeStats, kStatsHistoryCapacity> stats_history_;
 
+  intptr_t scavenge_words_per_micro_;
+  intptr_t idle_scavenge_threshold_in_words_;
+
   // The total size of external data associated with objects in this scavenger.
   intptr_t external_size_;