Update docs and command line flags for 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
 each bot builds (i.e., the configurations can be changed from chromium
 commits), and to consolidate the list of all of the various configurations
 that Chromium is built in.
 
 Ideally this tool will no longer be needed after the migration is complete.
 
-## `mb gen`
+For more discussion of MB, see also [the design spec](design_spec.md).
 
-`mb gen` is responsible for generating the Ninja files by invoking either GYP
-or GN as appropriate. It takes arguments to specify a build config and
-a directory, then runs GYP or GN as appropriate:
+## MB subcommands
 
-```
-% 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 "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`
 
 `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 must be a GN-style "source-absolute" path
 to the build directory.
 
 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 (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
       `targets` and `build_targets` are not returned).
 
-## `mb help`
+The `-b/--builder`, `-c/--config`, `-f/--config-file`, `-m/--master`,
+`-q/--quiet`, and `-v/--verbose` flags work as documented for `mb gen`.
+
+
+### `mb gen`
+
+`mb gen` is responsible for generating the Ninja files by invoking either GYP
+or GN as appropriate. It takes arguments to specify a build config and
+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.
+
+By default, MB will look in `//tools/mb/mb_config.pyl` to look up the config
+information, but you can specify a custom config file using the
+`-f/--config-file` flag.
+
+The path must be a GN-style "source-absolute" path (as above).
+
+You can pass the `-n/--dryrun` flag to mb gen to see what will happen without
+actually writing anything.
+
+You can pass the `-q/--quiet` flag to get mb to be silent unless there is an
+error, and pass the `-v/--verbose` flag to get mb to log all of the files
+that are read and written, and all the commands that are run.
+
+If the build config will use the Goma distributed-build system, you can pass
+the path to your Goma client in the `-g/--goma-dir` flag, and it will be
+incorporated into the appropriate flags for GYP or GN as needed.
+
+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 help`
 
 Produces help output on the other subcommands
 
-## `mb lookup`
+### `mb lookup`
 
 Prints what command will be run by `mb gen` (like `mb gen -n` but does
 not require you to specify a path).
 
-## `mb validate`
+The `-b/--builder`, `-c/--config`, `-f/--config-file`, `-m/--master`,
+`-q/--quiet`, and `-v/--verbose` flags work as documented for `mb gen`.
+
+### `mb validate`
 
 Does internal checking to make sure the config file is syntactically
 valid and that all of the entries are used properly. It does not validate
 that the flags make sense, or that the builder names are legal or
 comprehensive, but it does complain about configs and mixins that aren't
 used.
 
+The `-f/--config-file` and `-q/--quiet` flags work as documented for
+`mb gen`.
+
 This is mostly useful as a presubmit check and for verifying changes to
 the config file.
 
-## mb_config.pyl
+## Isolates and Swarming
+
+`mb gen` is also responsible for generating the `.isolate` and
+`.isolated.gen.json` files needed to run test executables through swarming
+in a GN build (in a GYP build, this is done as part of the compile step).
+
+If you wish to generate the isolate files, pass `mb gen` the
+`--swarming-targets-file` command line argument; that arg should be a path
+to a file containing a list of ninja build targets to compute the runtime
+dependencies for (on Windows, use the ninja target name, not the file, so
+`base_unittests`, not `base_unittests.exe`).
+
+MB will take this file, translate each build target to the matching GN
+label (e.g., `base_unittests` -> `//base:base_unittests`, write that list
+to a file called `runtime_deps` in the build directory, and pass that to
+`gn gen $BUILD ... --runtime-deps-list-file=$BUILD/runtime_deps`.
+
+Once GN has computed the lists of runtime dependencies, MB will then
+look up the command line for each target (currently this is hard-coded
+in [mb.py](https://code.google.com/p/chromium/codesearch?q=mb.py#chromium/src/tools/mb/mb.py&q=mb.py%20GetIsolateCommand&sq=package:chromium&type=cs)), and write out the
+matching `.isolate` and `.isolated.gen.json` files.
+
+## The `mb_config.pyl` config file
 
 The `mb_config.pyl` config file is intended to enumerate all of the
 supported build configurations for Chromium. Generally speaking, you
 should never need to (or want to) build a configuration that isn't
 listed here, and so by using the configs in this file you can avoid
 having to juggle long lists of GYP_DEFINES and gn args by hand.
 
 `mb_config.pyl` is structured as a file containing a single PYthon Literal
 expression: a dictionary with three main keys, `masters`, `configs` and
 `mixins`.
@@ skipping 66 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).
 
-## Debugging (-v/--verbose and -n/--dry-run)
+## 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.