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MemoryDumpManagerTest didn't join its thread. This made a write in
TearDown() racy with a read from a thread created by the test.

With this CL, threads created by MemoryDumpManagerTest are joined
before TearDown() runs.

BUG=658766

Review-Url: https://codereview.chromium.org/2447853002
Cr-Commit-Position: refs/heads/master@{#427251}