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And don't break presubmit!
Basically, the change I put in previously that kind of fixed extensions redirects
actually meant presubmit would fail.
So I've gone and put in a real fix.

I also updated the redirects.json files in these GitHub repos for this to work: devtools, multi-device, and store.

Thanks, Ben, for your help.

Review URL: https://codereview.chromium.org/883963002

Cr-Commit-Position: refs/heads/master@{#313575}