While feature flags generally allow us to remove features down the road, in either the chromium or support-lib side, there are some notable exceptions. Some features must never be removed (pending certain conditions).

Some examples:

 * SAFE_BROWSING_RESPONSE_SHOW_INTERSTITIAL: this is invoked by the default implementation of WebViewClientCompat#onSafeBrowsingHit()
 * WEB_MESSAGE_GET_DATA: consensus on this doc [1] seems to be to never feature check these methods, and to support them as long as any webmessage API is supported

We should consider a presubmit or unit test to enforce that we never remove these (and similar features). If we add a unit test, we should consider enforcing that the methods themselves exist as well (since the feature flag is redundant at this point).

[1] https://docs.google.com/document/d/1vovTjs3nKizkZCYGMINNNF2V6DqPEPQOHxyj-cjLIlQ/edit?usp=sharing