Minor simplification and cleanup of graph builder.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/448113002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23000 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix BUILD.gn after unflagging unscopables

TBR=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/455823002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23001 ce2b1a6d-e550-0410-aec6-3dcde31c8c00