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Change BlockableProxyResolver to use a single ConditionVariable rather than multiple WaitableEvents.

This simplification turns incorrect call sequences into a test failure rather than a data race.

Change-Id: I8333ba47fd269559f55da2252a8f76895beb571d
Reviewed-on: https://chromium-review.googlesource.com/988741Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547917}