[tools] Filter empty categories in GC eval script

tools/eval_gc_nvp.py

 #!/usr/bin/env python
 #
 # Copyright 2015 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.
 
 """This script is used to analyze GCTracer's NVP output."""
 
 
 from argparse import ArgumentParser
@@ skipping 79 matching lines @@
     return min(self.values)
 
   def max(self):
     return max(self.values)
 
   def avg(self):
     if len(self.values) == 0:
       return 0.0
     return sum(self.values) / len(self.values)
 
+  def empty(self):
+    return len(self.values) == 0
+
   def __str__(self):
     if self.csv:
       ret = [self.key]
       ret.append(len(self.values))
       ret.append(self.min())
       ret.append(self.max())
       ret.append(self.avg())
       ret = [str(x) for x in ret]
       return ",".join(ret)
     else:
@@ skipping 62 matching lines @@
                  for key in args.keys ]
 
   while True:
     line = stdin.readline()
     if not line:
       break
     obj = split_nvp(line)
     for category in categories:
       category.process_entry(obj)
 
+  # Filter out empty categories.
+  categories = [x for x in categories if not x.empty()]
+
   if args.rank != "no":
     categories = sorted(categories, key=make_key_func(args.rank), reverse=True)
 
   for category in categories:
     print(category)
 
 
 if __name__ == '__main__':
   main()