Magnetometer data doesn't make sense and doesn't match caniuse.com

[foolip]

https://github.com/web-platform-dx/web-features/blob/5f3d1e80a508b8ba0ec1e240f0ce4bc4acf7dbfd/features/magnetometer.yml.dist doesn't make sense, it is meaningless to say that <iframe allow="magnetometer"> or Permissions-Policy for the feature is supported when the feature itself isn't. Those code paths, even if active in the browser, wouldn't end up doing anything observable.

https://caniuse.com/magnetometer claims the whole feature is supported since Chrome 67, but I don't believe that.

In Chromium the features is controlled by a flag which is off by default:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/runtime_enabled_features.json5;l=3940-3943;drc=bdbfc851687b28b24b80bccbc4c027bfe05a701b

Testing on Chrome on macOS, the Magnetometer interface isn't exposed.

@anssiko @riju as editors of the spec, can you say something about the situation? Is the Magnetometer API enabled by default on some platform?

BCD, caniuse.com and web-features need updating to align on whatever the correct state is.

[anssiko]

Magnetometer is behind about://flags/#enable-generic-sensor-extra-classes in Chromium-based browsers. BCD et al. should be updated to reflect this. It looks like caniuse.com does note the flag for Chrome, but versions are off. Thanks for looking into this!

[ddbeck]

Leaving a cross-reference for myself: doing #1680 would catch this kind of problem in general.

[ddbeck]

I've sent mdn/browser-compat-data#25897 and Fyrd/caniuse#7259.