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It was using a single global worker thread pool in a non-threadsafe
manner.

This imposed the restriction that all instances of SimpleCache must be
initialized on a single thread, or weird things may, in very rare and
unlikely cases, occur.

BUG=442321

Review URL: https://codereview.chromium.org/851013003

Cr-Commit-Position: refs/heads/master@{#317071}