Make sure that there is enough time left before finishing incremental marking in idle notification.

src/gc-tracer.h

Index: src/gc-tracer.h
diff --git a/src/gc-tracer.h b/src/gc-tracer.h
index 0f4f135256ecc6ebd0ca0ea68ba1b68bf299108a..4802959e478db082762b78faf49ae3676eab739c 100644
--- a/src/gc-tracer.h
+++ b/src/gc-tracer.h
@@ -254,6 +254,11 @@ class GCTracer BASE_EMBEDDED {
   // no events have been recorded.
   intptr_t MarkingSpeedInBytesPerMillisecond() const;
 
+  // Returns the duration of the last incremental marking step.
+  double last_incremental_marking_step_duration() const {
+    return last_incremental_marking_step_duration_;
+  }
+
  private:
   // Print one detailed trace line in name=value format.
   // TODO(ernstm): Move to Heap.
@@ -298,6 +303,9 @@ class GCTracer BASE_EMBEDDED {
   // Cumulative duration of incremental marking steps since creation of tracer.
   double cumulative_incremental_marking_duration_;
 
+  // Duration of last incremental marking step.
+  double last_incremental_marking_step_duration_;
+
   // Longest incremental marking step since start of marking.
   double longest_incremental_marking_step_;