Convert ClipIds for Clips into indices in the ClipScrollTree

[mrobinson]

During display list flattening, we convert ClipIds for the clipping node
into ClipChainIndex (as well as allocating space for them in the
ClipChain array in the ClipScrollTree). This allows us to avoid doing a
HashMap lookup for the ClipChain for every primitive run. It also moves
us a little closer to a universal representation of ClipChains (both
API-defined ClipChains and ones derived from the hierarchy of the
ClipScroll tree).

This causes the amount of time spend doing HashMap lookups to decrease
in the profile, though they are still done for the positioning node. A
second patch will reduce HashMap lookups for positioning nodes as well.

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This change is 

[mrobinson]

Here is the Gecko try job for this change: https://treeherder.mozilla.org/#/jobs?repo=try&revision=29be18dfbcc84d5ab006266ca437a6d303ec1ebb

[glennw]

Reviewed 10 of 10 files at r1.
Review status: all files reviewed at latest revision, 3 unresolved discussions.

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webrender/src/frame_builder.rs, line 656 at r1 (raw file):

        self.reference_frame_stack.push(reference_frame_id);

        if let Some(ref parent_id) = parent_id {

match would probably be tidier here

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webrender/src/hit_test.rs, line 298 at r1 (raw file):

    flags: HitTestFlags,
    node_cache: FastHashMap<ClipId, Option<LayerPoint>>,
    clip_chain_cache: Vec<Option<bool>>,

This could potentially be a bitvec or something, I think. But it's no big deal - I can't imagine it ever showing up in a profile.

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webrender/src/render_task.rs, line 60 at r1 (raw file):

#[derive(Debug, Clone)]
pub struct ClipChain {

Not needed here - but it might make sense to move ClipChain to clip.rs as a follow up, since it's much more integral now?

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Comments from Reviewable

[glennw]

There's a couple of minor (optional) comments above, r=me either way. I'll leave it to you to merge when ready (not sure if you need to discuss with @nical any conflicts likely with the PR he's working on).

[bors-servo]

☔ The latest upstream changes (presumably #2424) made this pull request unmergeable. Please resolve the merge conflicts.

[mrobinson]

Review status: 3 of 12 files reviewed at latest revision, 3 unresolved discussions.

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webrender/src/frame_builder.rs, line 656 at r1 (raw file):

Previously, glennw (Glenn Watson) wrote…

match would probably be tidier here

Oh, yeah. That's a bit better. I've made the change.

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webrender/src/hit_test.rs, line 298 at r1 (raw file):

Previously, glennw (Glenn Watson) wrote…

This could potentially be a bitvec or something, I think. But it's no big deal - I can't imagine it ever showing up in a profile.

I'll save this one for later if that's okay.

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webrender/src/render_task.rs, line 60 at r1 (raw file):

Previously, glennw (Glenn Watson) wrote…

Not needed here - but it might make sense to move ClipChain to clip.rs as a follow up, since it's much more integral now?

This makes sense to me as well and is relatively harmless, so I've gone ahead and moved the code now.

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Comments from Reviewable

[mrobinson]

@bors-servo r=glennw

[bors-servo]

📌 Commit f11b8c7 has been approved by glennw

[bors-servo]

⌛ Testing commit f11b8c7 with merge d2de084...

[bors-servo]

💔 Test failed - status-travis

[mrobinson]

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

Looks like the build stalled so I will retry.

[mrobinson]

@bors-servo retry

[bors-servo]

⌛ Testing commit f11b8c7 with merge 41ae26b...

[bors-servo]

💔 Test failed - status-travis

[mrobinson]

@bors-servo retry

[bors-servo]

⌛ Testing commit f11b8c7 with merge ea86420...

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster, status-travis
Approved by: glennw
Pushing ea86420 to master...