Deletes bench_expectations.txt and change comments to refer to the new per-bot expectation files.

bench/gen_bench_ranges.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be found
 # in the LICENSE file.
 
 """ Analyze recent SkPicture or Microbench data, and output suggested ranges.
 
-The outputs can be edited and pasted to bench_expectations.txt to trigger
-buildbot alerts if the actual benches are out of range. Details are documented
-in the .txt file.
+The outputs can be edited and pasted to bench_expectations_<builder>.txt to
+trigger buildbot alerts if the actual benches are out of range. Details are
+documented in the corresponding .txt file for each builder.
 
-Currently the easiest way to batch update bench_expectations.txt is to delete
-all bench lines, run this script, and redirect outputs (">>") to be added to the
-.txt file.
+Currently the easiest way to batch update bench_expectations_<builder>.txt is to
+delete all bench lines, run this script, and redirect outputs (">>") to be added
+to the corresponding .txt file for each perf builder.
 You can also just manually change a few lines of interest, of course.
 
 Note: since input data are stored in Google Storage, you will need to set up
 the corresponding library.
 See http://developers.google.com/storage/docs/gspythonlibrary for details.
 """
 
 __author__ = 'bensong@google.com (Ben Chen)'
 
 import bench_util
@@ skipping 147 matching lines @@
     else:
       rev_min = int(range_match.group(1))
       rev_max = int(range_match.group(2))
       OutputBenchExpectations(options.bench_type, rev_min, rev_max, options.alg)
   else:
     parser.error('Please provide mandatory flag %s' % OPTION_REVISION_RANGE)
 
 
 if '__main__' == __name__:
   main()