Allow GN imports to be processed from imports.

This restriction is unnecessarily restrictive and I will need to be able to do
this in a future CL. Rather than prevent variable collisions, this instead
checks for value collisions so as long as you end up with the same value, the
error won't be thrown. This mainly comes up with multiple imports, so the
values will be the same.

R=piman@chromium.org

Review URL: https://codereview.chromium.org/103593006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241071 0039d316-1c4b-4281-b951-d872f2087c98