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There were two different watchers for WebContents destruction in two
different test helper files, in addition to lots of tests rolling their
own. Consolidate all of them.

Bug: none
Change-Id: I9b5723758aa997467689cce8767a3def9e8c13e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153747
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760054}