The IPCDataTransfer type is used to transfer Clipboard/Drag & Drop payloads
over IPC to allow them to be written to or read from the relevant system
interfaces. Previously, the system which was used was somewhat complex, and
tried to use Shmem in some cases to store buffers out of line. Now that
BigBuffer is available, it can be simplified substantially.

In addition, this change removed the memory buffer overload of GetSurfaceData,
as the only consumer was using it to immediately send the payload over IPC as a
nsCString. It was changed to instead use `BigBuffer` as that is more efficient
in a large buffer situation, and reduces the number of required copies.

Differential Revision: https://phabricator.services.mozilla.com/D151852