Use current time to calculate context disposal rate.

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 2655f1f48cb1e5c5af39389b4d82859c6c8ef6ee..3e6a8a77a920c4afa9ef2e596dc2652c0a751b9a 100644
--- a/src/heap/gc-tracer.h
+++ b/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;