1. Use GetWithoutInvalidation() instead of Get() in DCHECKs.
We should never call Get() inside of a DCHECK(), because this can
lead to a different code path depending on whether DCHECKs are enabled.

2. Get() should not cause immediate side effects. At most, it should
queue up an invalidation for later processing.

Fixing #1 and #2 were required in order to get past a first set of
errors introduced by the new test.

3. The actual fix -- avoid infinite loop by calling a special
new SlotAssignmentWillChange(), rather than ChildrenChanged(),
where a minimal GetWithoutInvalidation() is called that does not
lead to IsShadowContentRelevantForAccessibility() => FirstChild() =>
RecalcAssignedNodes() => ChildrenChanged() ... (infinite loop).

A simpler potential fix is in CL:2965317 but requires more
research. It's also mentioned in a TODO comment.

Bug: 1219311
Change-Id: Iafaa289f241a851404ce352715d2970172a2e5f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2961158
Reviewed-by: Joey Arhar <[email protected]>
Reviewed-by: Mason Freed <[email protected]>
Reviewed-by: Dominic Mazzoni <[email protected]>
Commit-Queue: Aaron Leventhal <[email protected]>
Cr-Commit-Position: refs/heads/master@{#892778}