Add factory functions to distinguish between when the owner of the File
wants to transfer ownership to the new IPC object (adopt) or to send a
copy of the same fd to the IPC peer (clone).
This behavior is more intuitive than the previous behavior. Previously,
an IPC::File would default to a shallow clone of the file descriptor,
only *actually* calling dup(2) for the fd when encoding or it into an
IPC MessageBuffer. Now the dup(2) for the fd is explicit in the clone_fd
factory function.
We had previous implemented some plumbing for file input elements in
commit 636602a54e.
This implements the return path for chromes to inform WebContent of the
file(s) the user selected. This patch includes a dummy implementation
for headless-browser to enable testing.
