MediaKeys.setServerCertificate() returns a promise that resolves to true or false. Rather than treating a rejection with NOTSUPPORTEDERROR as a resolve(false) [1], use a CdmPromise<bool> so that CDMs can signal if they support certificates or not.

[1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/encryptedmedia/MediaKeys.cpp?l=142