Add pre_log option to nanobench; make this option default true in nanobench and dm.

tools/nanobench_flags.py

 #
 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 
 #!/usr/bin/env python
 
 usage = '''
 Write extra flags to outfile for nanobench based on the bot name:
   $ python nanobench_flags.py outfile Perf-Android-GCC-GalaxyS3-GPU-Mali400-Arm7-Release
 Or run self-tests:
   $ python nanobench_flags.py test
 '''
 
 import inspect
 import json
 import os
 import sys
 
 
 def lineno():
   caller = inspect.stack()[1]  # Up one level to our caller.
   return inspect.getframeinfo(caller[0]).lineno
 
 
 cov_start = lineno()+1   # We care about coverage starting just past this def.
 def get_args(bot):
-  args = []
+  args = ['--pre_log']
 
   if 'GPU' in bot:
     args.append('--images')
     args.extend(['--gpuStatsDump', 'true'])
 
   if 'Appurify' not in bot:
     args.extend(['--scales', '1.0', '1.1'])
 
   if 'iOS' in bot:
     args.extend(['--skps', 'ignore_skps'])
@@ skipping 118 matching lines @@
   if len(sys.argv) == 2 and sys.argv[1] == 'test':
     self_test()
     sys.exit(0)
 
   if len(sys.argv) != 3:
     print usage
     sys.exit(1)
 
   with open(sys.argv[1], 'w') as out:
     json.dump(get_args(sys.argv[2]), out)