Implement multi-phase build support in 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 94 matching lines @@
 `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.
+must be specified so that `mb` can figure out which config to use. The
+`--phase` flag must also be used with builders that have multiple
+build/compile steps (and only with those builders).
 
 By default, MB will look for a bot config file under `//ios/build/bots` (see
 [design_spec.md](the design spec) for details of how the bot config files
 work). If no matching one is found, will then 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
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 ### `mb help`
 
 Produces help output on the other subcommands
 
 ### `mb lookup`
 
 Prints what command will be run by `mb gen` (like `mb gen -n` but does
 not require you to specify a path).
 
 The `-b/--builder`, `-c/--config`, `-f/--config-file`, `-m/--master`,
-`-q/--quiet`, and `-v/--verbose` flags work as documented for `mb gen`.
+`--phase`, `-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
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 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`.
 
 The `masters` key contains a nested series of dicts containing mappings
 of master -> builder -> config . This allows us to isolate the buildbot
-recipes from the actual details of the configs.
+recipes from the actual details of the configs. The config should either
+be a single string value representing a key in the `configs` dictionary,
+or a list of strings, each of which is a key in the `configs` dictionary;
+the latter case is for builders that do multiple compiles with different
+arguments in a single build, and must *only* be used for such builders
+(where a --phase argument must be supplied in each lookup or gen call).
 
-The `configs` key points to a dictionary of named build
-configurations.
+The `configs` key points to a dictionary of named build configurations.
 
 There should be an key in this dict for every supported configuration
 of Chromium, meaning every configuration we have a bot for, and every
 configuration commonly used by develpers but that we may not have a bot
 for.
 
 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.
 
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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.