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

tools/mb/docs/user_guide.md

Index: tools/mb/docs/user_guide.md
diff --git a/tools/mb/docs/user_guide.md b/tools/mb/docs/user_guide.md
index 7f002919aeae663b30d4339d48f79957a0c793bb..46e8c144c1899227122b23944edf4b2130d4f223 100644
--- a/tools/mb/docs/user_guide.md
+++ b/tools/mb/docs/user_guide.md
@@ -177,7 +177,7 @@ following keys:
   * `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
@@ -228,6 +228,17 @@ translate into a call to `gyp_chromium -G Release` with `GYP_DEFINES` set to
 (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.