Flesh out MojoUrlRedirector apptests

Flesh out MojoUrlRedirector apptests

This CL adds the following tests of the MojoUrlRedirector:
- When asked for an app whose location file is missing, produces 404 response.
- When asked for an app on a known platform, returns a redirect to the location
  contained in the app's location file for that platform.

To implement these tests, the MojoUrlRedirector now allows for parameterizing
the address that it asks for apps' location files from. The test spins up an
HttpServer on localhost, registers an HttpHandler that processes requests for
app location files, and instructs the MojoUrlRedirector to talk to that server
to obtain app location files.

R=ppi@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/b1c5dd67e3d3b64fccaac879ab33d4ab703a240a

[blundell, 2015-03-25 11:01:39]

blundell@chromium.org changed reviewers:
+ ppi@chromium.org

[ppi, 2015-03-30 12:15:16]

lgtm

As discussed off-bug, it would be interesting to explore asynchronous
redirection initialization, e.g. using  GetPort() that calls back only after the
server is fully set up).

[blundell, 2015-04-01 10:04:00]

Committed patchset #3 (id:40001) manually as
b1c5dd67e3d3b64fccaac879ab33d4ab703a240a (presubmit successful).

[qsr, 2015-04-01 13:49:03]

qsr@chromium.org changed reviewers:
+ qsr@chromium.org

[qsr, 2015-04-01 13:49:03]

https://codereview.chromium.org/1037733002/diff/40001/services/python/mojo_ur...
File services/python/mojo_url_redirector/__mojo__.py (right):

https://codereview.chromium.org/1037733002/diff/40001/services/python/mojo_ur...
services/python/mojo_url_redirector/__mojo__.py:50: custom_headers["location"] =
new_location
Usually Location (capital L) is used there.

https://codereview.chromium.org/1037733002/diff/40001/services/python/mojo_ur...
services/python/mojo_url_redirector/__mojo__.py:150: assert
len(application.args) >= 2
You might want to use argparse instead of place, this is quickly becoming
unmanageable...

https://codereview.chromium.org/1037733002/diff/40001/services/python/mojo_ur...
File services/python/mojo_url_redirector/mojo_url_redirector_apptest.cc (right):

https://codereview.chromium.org/1037733002/diff/40001/services/python/mojo_ur...
services/python/mojo_url_redirector/mojo_url_redirector_apptest.cc:84: for
(const std::string& arg : application_impl()->args()) {
Given that you are using base, you can probably set the command line using your
args, and then use all the parsing utility that we have to find the parameter
you need.

base::CommandLine::StringVector command_line_args = app->args();
base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
command_line->InitFromArgv(command_line_args);