[Feature Policy: sub-feature] Sandbox features #4402

The iframe sandbox attribute (caniuse, WHATWG spec) has proven to be too limiting, and Feature-Policy (caniuse, WICG draft spec) is going to inherit the features of sandbox - potentially making all sandbox directives obsolete except for the sandbox attribute itself.

For more information, see:

• Iframe Sandboxing with Feature Policy
• Migrating Sandbox attributes to Feature Policy

Chrome status:
https://www.chromestatus.com/feature/5232684104155136