The code to handle MediaStream URIs is almost the same as for Blobs, so share it.
nsHostObjectProtocolHandler is modified a bit to simplify method names. Also
we make nsHostObjectProtocolHandler::AddDataEntry take responsibility for
generating the URI to avoid duplicating that code later.
We need separate subclasses for each kind of host object protocol handler,
but we don't need separate subclasses for each kind of host object URI.
