Use current time to calculate context disposal rate.

src/heap/gc-tracer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/heap/gc-tracer.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 474 matching lines @@
 
   if (durations == 0.0) return 0;
 
   return static_cast<intptr_t>(bytes / durations);
 }
 
 
 double GCTracer::ContextDisposalRateInMilliseconds() const {
   if (context_disposal_events_.size() == 0) return 0.0;
 
-  double begin = context_disposal_events_.begin()->time_;
+  double begin = base::OS::TimeCurrentMillis();
   double end = 0.0;
   ContextDisposalEventBuffer::const_iterator iter =
       context_disposal_events_.begin();
   while (iter != context_disposal_events_.end()) {
     end = iter->time_;
     ++iter;
   }
 
   return (begin - end) / context_disposal_events_.size();
 }
 }
 }  // namespace v8::internal