Use current time to calculate context disposal rate.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/679363003

Cr-Commit-Position: refs/heads/master@{#25212}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25212 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap/gc-tracer.cc

@@ -492,7 +492,7 @@ intptr_t GCTracer::NewSpaceAllocationThroughputInBytesPerMillisecond() const {
 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();

src/heap/gc-tracer.h

@@ -341,8 +341,8 @@ class GCTracer {
   intptr_t NewSpaceAllocationThroughputInBytesPerMillisecond() const;
 
   // Computes the context disposal rate in milliseconds. It takes the time
-  // frame of the first and last context disposal event and devides it by the
-  // number of recorded events.
+  // frame of the first recorded context disposal to the current time and
+  // divides it by the number of recorded events.
   // Returns 0 if no events have been recorded.
   double ContextDisposalRateInMilliseconds() const;