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This adds special handling for variables that begin with underscores, which is inspired by Dart. Such variables are not imported when doing an import, which gives .gni files a way to have intermediate private variables that won't pollute the scopes of the files that include them.

This also applies to the root build config, which can have private values now.

Adds some missing unused variable checks. This was disabled because processing imports would mean all imported variables were unused, and files not using all of them would get unused variable errors. This adds the option to mark merged values on a scope as used, which is used for imported values.

BUG=341738
R=cjhopman@chromium.org

Review URL: https://codereview.chromium.org/287693002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271078 0039d316-1c4b-4281-b951-d872f2087c98