Remove redundant code, now that only local clips are segmented.

[gw3583]

As of #2859, this code path can never be executed. Removing it
also simplifies some of the code I'm working on for local space
picture rasterization.

---

This change is 

[gw3583]

r? @mrobinson or @kvark

Pending try:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=8bf7fcf0c5a0ddcdbf8e59be6396355782a1168f

[mrobinson]

@bors-servo r+

[bors-servo]

📌 Commit 9f011c9 has been approved by mrobinson

[bors-servo]

⌛ Testing commit 9f011c9 with merge e600bfe...

[gw3583]

Holy cow, that's a lot of orange on the try run! @staktrace Is this a known issue on try runs right now, or did I somehow manage to break things?

[gw3583]

A lot of the errors are:

'internal error: entered unreachable code: Got a non Clip ClipId for root reference frame.', gfx/webrender_bindings/src/bindings.rs:2444:14

The code I've changed here is clip related, but I can't see how it would cause that. Maybe the bindings are out of sync or something like that?

[gw3583]

Ah, I think the following code in bindings.rs probably needs to be updated after #2871 and #2883.

#[no_mangle]
pub extern "C" fn wr_root_scroll_node_id() -> usize {
    // The PipelineId doesn't matter here, since we just want the numeric part of the id
    // produced for any given root reference frame.
    match ClipId::root_scroll_node(PipelineId(0, 0)) {
        ClipId::Clip(id, _) => id,
        _ => unreachable!("Got a non Clip ClipId for root reference frame."),
    }
}

Is that right @mrobinson ?

[mrobinson]

@gw3583 The changes necessary on the Gecko side for splitting the CipScrollTree were actually non-trivial. Do you mind applying the commit from my try job [1] for that PR and then trying the try job again?

1. https://hg.mozilla.org/try/rev/33639cd81873baf1c23e6e061900ea3433e4fec1

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: mrobinson
Pushing e600bfe to master...

[gw3583]

@mrobinson Ah, sure - I'll apply that patch and do a new try run. Thanks!

[gw3583]

Pending try with that patch:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=f77c2ce082b48239229d7d917f3af7ad2bcaafd5

[mrobinson]

The try job look pretty good to me. There are a couple crashes, but I'm not sure that they are related.