When ReadSegments is called before the stream is ready, we return NS_BASE_STREAM_WOULD_BLOCK, not NS_ERROR_NOT_IMPLEMENTED.
If callers use AsyncWait for the IPCBlobInputStream to be ready, and then call ReadSegments again we can end up returning NS_ERROR_NOT_IMPLEMENTED, depending on what the inner type is.
Differential Revision: https://phabricator.services.mozilla.com/D51781
--HG--
extra : moz-landing-system : lando
