Testing and bugfixing for the new MB gyp/gn wrapper.

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
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 a directory, then runs GYP or GN as appropriate:
 
 ```
 % mb gen -m tryserver.chromium.linux -b linux_rel //out/Release
 % mb gen -c linux_rel_trybot //out/Release
 ```
 
 Either the `-c/--config` flag or the `-m/--master` and `-b/--builder` flags
 must be specified so that `mb` can figure out which config to use.
 
-The path must be a GN-style path relative to the checkout root (as above).
+The path must be a GN-style "source-absolute" path (as above).
 
 If gen ends up using GYP, the path must have a valid GYP configuration as the
 last component of the path (i.e., specify `//out/Release_x64`, not `//out`).
 
 ## `mb analyze`
 
 `mb analyze` is reponsible for determining what targets are affected by
 a list of files (e.g., the list of files in a patch on a trybot):
 
 ```
 mb analyze -c chromium_linux_rel //out/Release input.json output.json
 ```
 
 Either the `-c/--config` flag or the `-m/--master` and `-b/--builder` flags
 must be specified so that `mb` can figure out which config to use.
 
 
-The first positional argument is the path to the build directory.
+The first positional argument must be a GN-style "source-absolute" path
+to the build directory.
 
-The second positional argument is a path to a JSON file containing a single
-object with two fields:
+The second positional argument is a (normal) path to a JSON file containing
+a single object with two fields:
 
   * `files`: an array of the modified filenames to check (as
     paths relative to the checkout root).
   * `targets`: an array of the unqualified target names to check.
 
-The third positional argument is a path where the mb will write the result,
-also as a JSON object. This object may contain the following fields:
+The third positional argument is a (normal) path to where mb will write
+the result, also as a JSON object. This object may contain the following
+fields:
 
   * `error`: this should only be present if something failed.
   * `targets`: the subset of the input `targets` that depend on the
     input `files`.
   * `build_targets`: the minimal subset of targets needed to build all
     of `targets` that were affected.
   * `status`: one of three strings:
     * `"Found dependency"` (build the `build_targets`)
     * `"No dependency"` (i.e., no build needed)
     * `"Found dependency (all)"` (build everything, in which case
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 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.