MB was originally designed to be a transitional tool for the GYP->GN migration.

Now that Chromium is fully on GN, we should figure out if we can get rid of it
and remove some layers and logic.

However, MB has grown to be useful in its own right and so we should figure out
what we need to replace it with.

It currently does the following things, all of which are useful:

1) manage which GN args should be used on which builder

2) manages a Ninja compile target <-> GN label mapping needed for analyze

3) manages the command lines needed for isolates on desktop platforms in conjunction
  with the //testing/buildbot/*.json files.

4) creating the *.isolated.gen.json files for the requested list of swarming targets
  (by passing the list of labels to `gn gen` to generate .runtime_deps files)

5) providing a slightly more friendly command line interface to //tools/swarming_client
  for generating and running isolates locally.

It's not yet clear what the best way to replace these things is. Some thoughts:

1) could be replaced with args files per builder or merged into JSON files like we do on iOS.

2) The recipes, JSON files, and script wrappers could be switched to specifying the ninja targets
   that more directly map to the labels (e.g., ui/base:ui_base_unittests instead of ui_base_unitttests).

3) The command lines can be pushed into GN itself (e.g., as an `args` variable in the test() template)
   and then written to a wrapper script (like out/Default/bin/run_ui_base_unittests), as we currently
   do on Android.

4) We could potentially just generate the .runtime_deps files all the time for any test() binary.

5) We could make the generated wrappers handle the isolating and swarming as well.