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As requested @ https://chromium-review.googlesource.com/c/chromium/src/+/738469/8/docs/design/threading.md#b122

Already had "Keep the Browser Responsive" and "Use Sequences instead of Locks"
sections, but amended them with a bit more text from the old docs.

R=jam@chromium.org

Bug: 752144
Change-Id: I67857ad21f8cc5884aeb65fe08fe184f63deed44
Reviewed-on: https://chromium-review.googlesource.com/921041Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537307}