Add a basic gn file for V8

Add a basic gn file for V8

Currently, the gn support is incomplete and only works from within a
chromium checkout.

See https://code.google.com/p/chromium/wiki/gn for details

We move the BUILD.gn file to v8 nevertheless to make renaming files
easier, as the chromium CQ already now checks the gn build.

BUG=none
R=brettw@chromium.org, machenbach@chromium.org
LOG=y

Committed: https://code.google.com/p/v8/source/detail?r=21140

[jochen (gone - plz use gerrit), 2014-04-29 15:05:02]

PTAL

Will it be possible to have a root group in that file as well to support
standalone builds at some point?

It's still not working on android, however, mksnapshot now links

[brettw, 2014-04-29 17:09:46]

LGTM!

https://codereview.chromium.org/259233002/diff/1/BUILD.gn
File BUILD.gn (right):

https://codereview.chromium.org/259233002/diff/1/BUILD.gn#newcode35
BUILD.gn:35: # TODO(jochen): Do actions need visibility settings as well?
I think it's a good idea. I'd just use ":*" for everything you don't want
external targets to depend on.

[Michael Achenbach, 2014-05-05 10:25:07]

Rubberstamp lgtm.

[jochen (gone - plz use gerrit), 2014-05-05 11:06:42]

Committed patchset #1 manually as r21140 (presubmit successful).