Make MB aware of iOS bot configs and get iOS working.

tools/mb/docs/design_spec.md

 # The MB (Meta-Build wrapper) design spec
 
 [TOC]
 
 ## Intro
 
 MB is intended to address two major aspects of the GYP -> GN transition
 for Chromium:
 
 1. "bot toggling" - make it so that we can easily flip a given bot
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 ### Configurations
 
 `mb` looks in the `//tools/mb/mb_config.pyl` config file to determine whether
 to use GYP or GN for a particular build directory, and what set of flags
 (`GYP_DEFINES` or `gn args`) to use.
 
 A config can either be specified directly (useful for testing) or by specifying
 the master name and builder name (useful on the bots so that they do not need
 to specify a config directly and can be hidden from the details).
 
+MB can also be configured to take the list of GYP_DEFINES and gn args from
+the caller if necessary, using the "passthrough" mode.
+
 See the [user guide](user_guide.md#mb_config.pyl) for details.
 
 ### Handling the analyze step
 
 The interface to `mb analyze` is described in the
 [user\_guide](user_guide.md#mb_analyze).
 
 Since the interface basically mirrors the way the "analyze" step on the bots
 invokes gyp\_chromium today, when the config is found to be a gyp config,
 the arguments are passed straight through.
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 * Some common flags (goma\_dir being the obvious one) may need to be
   specified via the user, and it's unclear how to integrate this with
   the concept of build\_configs.
 
   Right now, MB has hard-coded support for a few flags (i.e., you can
   pass the --goma-dir flag, and it will know to expand "${goma\_dir}" in
   the string before calling out to the tool. We may want to generalize
   this to a common key/value approach (perhaps then meeting the
   ChromeOS non-goal, above), or we may want to keep this very strictly
   limited for simplicity.