Add presubmit to run `{dm,nanobench}_flags.py test` when changed.

tools/dm_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 = '''
@@ skipping 156 matching lines @@
     args.extend(match)
 
   return args
 cov_end = lineno()   # Don't care about code coverage past here.
 
 
 def self_test():
   import coverage  # This way the bots don't need coverage.py to be installed.
   args = {}
   cases = [
+    'Pretend-iOS-Bot',
     'Test-Android-GCC-Nexus9-GPU-TegraK1-Arm64-Debug',
     'Test-Android-GCC-GalaxyS3-GPU-Mali400-Arm7-Debug',
+    'Test-Android-GCC-GalaxyS4-GPU-SGX544-Arm7-Release',
     'Test-Android-GCC-Nexus7-GPU-Tegra3-Arm7-Release',
     'Test-Android-GCC-NexusPlayer-CPU-SSSE3-x86-Release',
     'Test-Ubuntu-GCC-ShuttleA-GPU-GTX550Ti-x86_64-Release-Valgrind',
     'Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN',
     'Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Valgrind',
     'Test-Win7-MSVC-ShuttleA-GPU-HD2000-x86-Debug-ANGLE',
   ]
 
   cov = coverage.coverage()
   cov.start()
@@ skipping 17 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)