Don't add code dependencies eagerly for HCheckMaps.

Instead of adding code dependencies on stable during
graph creation, we now add them during code generation
for those HCheckMaps that survived dead code elimination.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/264973013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21139 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
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

Review URL: https://codereview.chromium.org/259233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21140 ce2b1a6d-e550-0410-aec6-3dcde31c8c00