Move tools/mb -> tools-webrtc/mb

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)