Blob::bytes() returns the ReadonlyBytes representation of our internal
ByteBuffer.
This change requires us to ByteBuffer::copy() Blob's ReadonlyBytes to a
ByteBuffer in XHR::send() and corresponding error handling of the copy
operation.
This removes the need for Blob to declare XMLHttpRequest as a friend
class.
This is a minor refactor of IDL::get_buffer_source_copy() letting it
return ErrorOr<ByteBuffer> instead of Optional<ByteBuffer>.
This also updates all places that use IDL::get_buffer_source_copy().
