Bug 1673349 [wpt PR 26284] - [scroll-animations] Handle element-based offsets with...

Bug 1673349 [wpt PR 26284] - [scroll-animations] Handle element-based offsets with null targets, a=testonly

Automatic update from web-platform-tests
[scroll-animations] Handle element-based offsets with null targets

Currently we assume that element-based offsets always have non-null
targets ResolveOffset-time, since we enforce this during the creation
of the (non-CSS) ScrollTimelines.

However, for CSSScrollTimelines, we can't invalidate the rule this way,
since we're referring to an element indirectly via an ID, not with a
pointer to a specific element.

Checking whether or not the target is null during ResolveOffset fixes
this, and it's also what we're supposed to be doing per spec.

This fixes a crash when using @scroll-timelines with element-based
offsets that reference non-existent IDs.

Also removed a nearby comment that's outdated (but not strictly
related to the change in this CL).

Bug: 1074052, 1102788
Change-Id: I4e138b55c3ef8cb122c58755a72a586e94c97e69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2498463
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821682}

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wpt-commits: 305dadc1abc586626e8605a3f9301b46f8baa4bb
wpt-pr: 26284