`gecko`'s browser/base/metrics.yaml should belong to `firefox_desktop`

[chutten]

See discussion in #376, but in summary: Metrics from browser/base/metrics.yaml made it into org_mozilla_firefox via the dependency on gecko, but that metrics file will contain only metrics that are sent as part of firefox_desktop (ie, it belongs to the browser Firefox Desktop alone, not the rendering engine Gecko which is shared with our Android browsers)

It would be tidier if this could be avoided.