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I hit a DCHECK using the frame tree node code path in the devtools code.
I audited the uses of frame tree node, and the two cases that could
possibly DCHECK or AV are these two. It makes sense that there is no way
to access the dev tools code when the fetch is not tied to a frame.

Change-Id: I571b77a4b4934e94ab5f6cf72b7aacdb0a71c67d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2606289Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840235}