GN: add default target.

Previously, invoking ninja without specifying a target would cause it to try to
build a number of unused and broken targets carried over from Chrome.  This was
worked around by building an explicit "root" target that contained only the
targets needed by Mojo.  The default (no target) build is now equivalent to
"root".  The "root" target still exists, but is now deprecated.

BUG=https://code.google.com/p/chromium/issues/detail?id=401761
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/809583002