Implement mb - a meta-build wrapper for bots to use in the GYP->GN migration.

tools/mb/README.md

+# MB - The Meta-Build wrapper
+
+MB is a simple wrapper intended to provide a uniform interface to either
+GYP or GN, such that users and bots can call one script and not need to
+worry about whether a given bot is meant to use GN or GYP.
+
+It supports two main functions:
+
+1. "gen" - the main `gyp_chromium` / `gn gen` invocation that generates the
+   Ninja files needed for the build.
+
+2. "analyze" - the step that takes a list of modified files and a list of
+   desired targets and reports which targets will need to be rebuilt.
+
+We also use MB as a forcing function to collect all of the different 
+build configurations that we actually support for Chromium builds into
+one place, in `//tools/mb/mb_config.pyl`.
+
+For more information, see:
+
+* [the user guide](docs/user_guide.md)
+* [the design spec](docs/design_spec.md)