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

tools/mb/docs/user_guide.md

 # The MB (Meta-Build wrapper) user guide
 
 [TOC]
 
 ## Introduction
 
 `mb` is a simple python wrapper around the GYP and GN meta-build tools to
 be used as part of the GYP->GN migration.
 
 It is intended to be used by bots to make it easier to manage the configuration
@@ skipping 159 matching lines @@
 value of the `mixins` key.
 
 Each mixin value is itself a dictionary that contains one or more of the
 following keys:
 
   * `gyp_crosscompile`: a boolean; if true, GYP_CROSSCOMPILE=1 is set in
     the environment and passed to GYP.
   * `gyp_defines`: a string containing a list of GYP_DEFINES.
   * `gn_args`: a string containing a list of values passed to gn --args.
   * `mixins`: a list of other mixins that should be included.
-  * `type`: a string with either the value `gyp` or `gn`;
+  * `type`: a string with either the value `gn`, `gyp`, or `passthrough`;
     setting this indicates which meta-build tool to use.
 
 When `mb gen` or `mb analyze` executes, it takes a config name, looks it
 up in the 'configs' dict, and then does a left-to-right expansion of the
 mixins; gyp_defines and gn_args values are concatenated, and the type values
 override each other.
 
 For example, if you had:
 
 ```
@@ skipping 30 matching lines @@
 }
 ```
 
 and you ran `mb gen -c linux_release_trybot //out/Release`, it would
 translate into a call to `gyp_chromium -G Release` with `GYP_DEFINES` set to
 `"use_goma=true dcheck_always_on=false dcheck_always_on=true"`.
 
 (From that you can see that mb is intentionally dumb and does not
 attempt to de-dup the flags, it lets gyp do that).
 
+If `type` is set to `passthrough`, then MB will look for the variable
+`MB_TYPE` in the environment, which should be set to either `gyp` or
+`gn`. If it is set to `gyp`, MB will also look for the GYP_DEFINES
+and GYP_CROSSCOMPILE variables in the environment. If it is set to `gn`,
+MB will look for the `GN_ARGS` variable in the environment. MB will
+throw away whatever values were set by the mixins and replace them
+with the values from the environment variables. This functionality exists
+in order to make bots that are otherwise already configured by src-side
+files happy. It is an error if GYP_DEFINES or GN_ARGS is expected to
+be set and isn't; GYP_CROSSCOMPILE is optional.
+
 ## Debugging MB
 
 By design, MB should be simple enough that very little can go wrong.
 
 The most obvious issue is that you might see different commands being
 run than you expect; running `'mb -v'` will print what it's doing and
 run the commands; `'mb -n'` will print what it will do but *not* run
 the commands.
 
 If you hit weirder things than that, add some print statements to the
 python script, send a question to gn-dev@chromium.org, or
 [file a bug](https://crbug.com/new) with the label
 'mb' and cc: dpranke@chromium.org.