Deflake MultiThreadedProxyResolverTest.ThreeThreads_Basic.

Deflake MultiThreadedProxyResolverTest.ThreeThreads_Basic.

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
Committed: https://crrev.com/3418083e501f3bf267508563b3f1762c1f22d5e3
Cr-Commit-Position: refs/heads/master@{#329360}

[Sam McNally, 2015-05-11 03:44:31]

Patchset #1 (id:1) has been deleted

[Sam McNally, 2015-05-11 08:22:50]

sammc@chromium.org changed reviewers:
+ eroman@chromium.org

[eroman, 2015-05-12 05:26:58]

lgtm

[Sam McNally, 2015-05-12 05:58:45]

The CQ bit was checked by sammc@chromium.org

[commit-bot: I haz the power, 2015-05-12 05:59:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1132943003/20001

[commit-bot: I haz the power, 2015-05-12 06:04:24]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-05-12 06:05:15]

Patchset 1 (id:??) landed as
https://crrev.com/3418083e501f3bf267508563b3f1762c1f22d5e3
Cr-Commit-Position: refs/heads/master@{#329360}