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The Karma tests that run do not currently produce a coverage report.
This CL adds local coverage reporting with a view to start exposing the
reports at the CQ level eventually.

Change-Id: Id374ee34c911a5762ce37ad69799fe00bcf1c29d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1831760
Commit-Queue: Paul Lewis <aerotwist@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701116}