Add Feature Policy integration for WebMidi

[raymeskhoury]

No description provided.

[raymeskhoury]

@cwilso @toyoshim ptal :)

[toyoshim]

LGTM

[toyoshim]

Shall we say that it causes a "SecurityError" in the requestMIDIAccess's algorithm, step 11?
In current specs even with this change, it's still unclear what happens if the feature is disabled by the policy.

[raymeskhoury]

Updated, ptal

[toyoshim]

Thanks! LGTM again.

[raymeskhoury]

Thanks - I have no write access to the repo - so if this should be merged, please merge this :)