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 dom@chromium.org
Reviewed-by: Joey Arhar jarhar@chromium.org
Cr-Commit-Position: refs/heads/main@{#1348159}