Reset old generation limit on main frame context disposal.

src/heap/gc-tracer.h

Index: src/heap/gc-tracer.h
diff --git a/src/heap/gc-tracer.h b/src/heap/gc-tracer.h
index a2e4bc293eefc4366d500fcc81ea945c828a33b0..528eb52c6455f073f8ed4139278ac4f2ef7462a1 100644
--- a/src/heap/gc-tracer.h
+++ b/src/heap/gc-tracer.h
@@ -71,6 +71,11 @@ class RingBuffer {
     elements_[begin_] = element;
   }
 
+  void reset() {
+    begin_ = 0;
+    end_ = 0;
+  }
+
  private:
   T elements_[MAX_SIZE + 1];
   size_t begin_;
@@ -390,6 +395,9 @@ class GCTracer {
   // Returns true if at least one survival event was recorded.
   bool SurvivalEventsRecorded() const;
 
+  // Discard all recorded survival events.
+  void ResetSurvivalEvents();
+
  private:
   // Print one detailed trace line in name=value format.
   // TODO(ernstm): Move to Heap.