Fix scaffold to work for the toolbar.
Fix scaffold to work for the toolbar.

Also, make the scaffold code more generic (with slots in a map,
instead of dedicated members for each slot).

This may eventually benefit from being split into a general
"multislot" superclass and a scaffold-specific subclass, but for now
it'll do.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1152163007

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