Fix nested config in GN.

This also moves the config out of the template. Otherwise this will generate an
error if there is more than one MIDL target in the same directory.

Use forward_variables_from in the MIDL template.

TBR=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1335283002

Cr-Commit-Position: refs/heads/master@{#348664}
2 files changed