Intent-to-Unship: DH algorithm support for WebCrypto

[J.C. Jones]

Our WebCrypto implementation supports using DH as an algorithm in
generateKey, which is not one of the recognized algorithms in the
published specification [0]. It doesn't even appear on MDN [2].

I intend to remove it from Firefox. However, before I do that, I am
landing telemetry [1] to determine whether it’s seeing use, at least
among our Nightly population. Given the relatively small sample, if we
get very low usage, that tells us nothing. However, if we do see
significant usage, we’ll have to be more careful in our plans.

My guess, since DH support is Firefox-only, is that after gathering
telemetry in the 68 cycle, we will unship in 69, unless further
information from Beta 68 causes us to revise course. I’ll plan to
update this thread as we learn more.

[0] https://www.w3.org/TR/WebCryptoAPI/#algorithm-overview
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1539578
[2] https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/generateKey