This adds a top-level BUILD.gn file and defines it as the root (by
removing the root_build override in //.gn) and adds forwarding rules to
the top-level target definitions in //build/gn/BUILD.gn. Also moves the
file generation steps to the top level BUILD.gn file.

The other rules in //build/gn/BUILD.gn should move too, but they are
currently referred to by infrastructure configuration files. They can
move after this lands and rolls to GI and the infra configs are updated.

Change-Id: I2043cc507944158cfe691db884d9fc79c2e80cc4

These directories appear to be --check clean now. Might as well turn on
the checking to keep them that way.

Change-Id: I27bd6e0880e67add1e960b94bba2fad95f35fa43

This patch neutralize the dot_gn_symlink.sh file and replace the
.gn symlink with the content of build/gn/dotfile.gn

Test: CQ
Change-Id: I229c338d34c1393ecd662fbc9aa04c039429220e

This patch add .gn symlink in fuchsia.git

Change-Id: I648fbdb4f294bb6bd6bd5d3cf8f95fd6a96d8bca

Change-Id: I374835029a93fc5cb16faefa8a1b75c97dfb72bf

Change-Id: Icb83422cd6473e152afb18a4d33be397dca2eb5f

All the build files are currently symlinked into mojo repo (../src)
except this file, which was copied from the mojo repo and modified:

mojo/tools/mojob.py
(sha1: dd752cf0606cbac8d9bc68c9752e0de27b124686)

Change-Id: I217631719c0c720afcf39bfa9413d3431216a649

Change-Id: Ie683f8678fc095c7a18fb93be0c660cbafb908cf

Change-Id: I19a95ad623fa24186be2ce8c57bdb0c361c4d761

Change-Id: I008d6441debb8bc1cbab65fb2d1308168f5ee7bd

Change-Id: I4a3389ddcee14d34820a7afd89667179b82ab64d

Change-Id: If979d307a4558f90466db094b2052ece9ab1df4a