Fix `mojo_run` to not require inferred adb path when running on Linux.

mojo/devtools/common/mojo_run

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 import argparse
 import logging
 import sys
 
 from devtoolslib.android_shell import AndroidShell
@@ skipping 65 matching lines @@
   # Desktop-only arguments.
   parser.add_argument('--use-osmesa', action='store_true',
                       help='(linux-only) Configure the native viewport service '
                       'for off-screen rendering.')
 
   launcher_args, args = parser.parse_known_args()
   mojo_paths, _ = paths.infer_mojo_paths(launcher_args.android,
                                          launcher_args.debug,
                                          launcher_args.target_cpu)
   if mojo_paths:
-    adb_path = mojo_paths['adb']
+    if launcher_args.android:
+      adb_path = mojo_paths['adb']
     shell_binary_path = mojo_paths['shell']
     local_origin_path = mojo_paths['build']
     if launcher_args.verbose:
       print 'Running within a Mojo checkout:'
       print ' - using the locally built shell at ' + shell_binary_path
       print ' - using the default origin of ' + local_origin_path
   else:
     if launcher_args.android:
       adb_path = 'adb'
       shell_binary_path = None
@@ skipping 48 matching lines @@
 
   if launcher_args.verbose:
     print "Shell arguments: " + str(args)
 
   shell.Run(args)
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())