Add support for the GYP_CROSSCOMPILE env var to MB.

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 156 matching lines @@
 
 The value of each key is a list of "mixins" that will define what that
 build_config does. Each item in the list must be an entry in the dictionary
 value of the `mixins` key.
 
 Each mixin value is itself a dictionary that contains one or more of the
 following keys:
 
   * `gyp_configs`: a list of the configurations to build, e.g.,
     ['Release', 'Release_x64'].
+  * `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`;
     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 type and
 gyp_configs values override each other.
@@ skipping 49 matching lines @@
 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.