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This change records UMA about how often *.googlezip.net proxies are
found and removed from the effective proxy configuration.

This will allow us to recognize when the temporary fix to remove those
proxies from the proxy config is no longer necessary, and can be safely
deleted.

BUG=488208

Review URL: https://codereview.chromium.org/1145513004

Cr-Commit-Position: refs/heads/master@{#330131}