Let marking speed depend on the actual incremental write barrier count.

Review URL: https://chromiumcodereview.appspot.com/11441011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13152 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/incremental-marking.cc

@@ -881,7 +881,7 @@ void IncrementalMarking::Step(intptr_t allocated_bytes,
   // allocation), so to reduce the lumpiness we don't use the write barriers
   // invoked since last step directly to determine the amount of work to do.
   intptr_t bytes_to_process =
-      marking_speed_ * Max(allocated_, kWriteBarriersInvokedThreshold);
+      marking_speed_ * Max(allocated_, write_barriers_invoked_since_last_step_);
   allocated_ = 0;
   write_barriers_invoked_since_last_step_ = 0;