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Now that only 1 read or write operation should be in progress at any
given time, add tests to verify that a second operation fails.

Bug: 958294
Test: new content_unittests pass
Change-Id: If34f667acd8094755bdc21027c966a61b783c90e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822499
Commit-Queue: John Rummell <jrummell@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699551}