Having two maps is probably excessive, since most users won't have that many
sidebars open, and keeping everything up to date is a bit of a pain. I'd just
have one map, keyed on web contents, and then if you need to look up a
particular sidebar, you can just traverse it (since it will be small).
https://codereview.chromium.org/1168383002/diff/60001/chrome/browser/extensions/sidebar_manager.h#newcode95