Bug 1915450 [wpt PR 47844] - DOM: Implement the Observable#reduce()…

… operator, a=testonly

Automatic update from web-platform-tests
DOM: Implement the `Observable#reduce()` operator

This CL specs the `reduce()` operator to have the same semantics as
the `reduce()` operator in TC39's iterator helpers proposal. See
WICG/observable#171.

R=jarhar

Bug: 40282760
Change-Id: I564b8cf0cf1fef81c0ee42808fad0a5aaddd2fc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5817639
Commit-Queue: Dominic Farolino <domchromium.org>
Reviewed-by: Joey Arhar <jarharchromium.org>
Cr-Commit-Position: refs/heads/main{#1348159}

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wpt-commits: 2a7b7ed6c088ae592ea3a1aa4437728596648cb8
wpt-pr: 47844

UltraBlame original commit: a0a2d5d15343b7dd8ba7b31bd8b0114df5e869a1