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Previously, MultiThreadedProxyResolverTest.ThreeThreads_Basic started
multiple requests in parallel and expected resolvers on three threads
to be created and used. However, the new resolver thread can take long
enough to become ready that it doesn't finish initializing its resolver
before all requests complete on other threads and the test finishes.
This CL fixes this by blocking the resolvers to force requests to run on
new resolver threads.

BUG=486257

Review URL: https://codereview.chromium.org/1132943003

Cr-Commit-Position: refs/heads/master@{#329360}