Fix bug where debugger.mojo not found in some build configurations

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 import shell_arguments
@@ skipping 15 matching lines @@
 
 The value of <handlers> is a comma separated list like:
 text/html,mojo:html_viewer,application/javascript,mojo:js_content_handler
 """
 
 # Port on which the mojo:debugger http server will be available on the host
 # machine.
 _MOJO_DEBUGGER_PORT = 7777
 _LAUNCHER = 'mojo:launcher'
 
-_DEBUGGER_URL = 'https://core.mojoapps.io/debugger.mojo'
+_DEBUGGER_URL = 'mojo:debugger'
 
 
 def _configure_debugger(shell):
   """Configures debugger.mojo to run and sets up port forwarding for its http
   server if the shell is running on a device.
 
   Returns:
     Arguments that need to be appended to the shell argument list in order to
     run with the debugger.
   """
@@ skipping 36 matching lines @@
 
   if script_args.verbose:
     print "Shell arguments: " + str(shell_args)
 
   shell.run(shell_args)
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())