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Add some DCHECK()s in the code to validate some assumptions (e.g. make
sure the reporters are terminated predictably), and update tests
accordingly.

BUG=none

Change-Id: Id01bd571ad1d59b90c297f71097edcf3e6edc59b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128448
Auto-Submit: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760220}