Chromium already sets this variable in common.gypi which is force included to all gyp files in the build.

Review URL: http://codereview.chromium.org/6250025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6433 ce2b1a6d-e550-0410-aec6-3dcde31c8c00