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Add document on types of threads that execute inside ChromeDriver,
and how these threads interact with each other.

Notry: true
Change-Id: Ib88095005062efe6189ffc9d2a6e391be8ffe1a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146929Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759259}