Make android_webview commits only try the android bot.

PRESUBMIT.py

 # 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 561 matching lines @@
   results.extend(_CheckSubversionConfig(input_api, output_api))
   return results
 
 
 def GetPreferredTrySlaves(project, change):
   files = change.LocalPaths()
 
   if not files:
     return []
 
-  if all(re.search('\.(m|mm)$|[/_]mac[/_.]', f) for f in files):
+  if all(re.search('\.(m|mm)$|(^|[/_])mac[/_.]', f) for f in files):
     return ['mac_rel', 'mac_asan']
-  if all(re.search('[/_]win[/_.]', f) for f in files):
+  if all(re.search('(^|[/_])win[/_.]', f) for f in files):
     return ['win_rel']
-  if all(re.search('[/_]android[/_.]', f) for f in files):
+  if all(re.search('(^|[/_])android[/_.]', f) for f in files):
     return ['android']
 
   trybots = ['win_rel', 'linux_rel', 'mac_rel', 'linux_clang:compile',
              'linux_chromeos', 'android', 'mac_asan']
 
   # Match things like path/aura/file.cc and path/file_aura.cc.
   # Same for ash and chromeos.
   if any(re.search('[/_](ash|aura)', f) for f in files):
     trybots += ['linux_chromeos', 'linux_chromeos_clang:compile', 'win_aura',
                 'linux_chromeos_asan']
   else:
     if any(re.search('[/_]chromeos', f) for f in files):
       trybots += ['linux_chromeos', 'linux_chromeos_clang:compile',
                   'linux_chromeos_asan']
 
   return trybots