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This allows us to take advantage of the corresponding contributed externs
file rather than roll our own.

This is not the latest release of chai.js (4.2.0 is), but it's the one
that has an available externs file.

Bug: 1000989
Change-Id: I7045efdcfbc12025b6857178c11fc5195d8dbaed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888619
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713343}