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Use the ChromeNetworkDelegate::OnProxyFallback() callback to keep track of number of times the data reduction proxies fall back because of network errors. Display a warning to users if there are network errors when connecting to the proxy and no successful requests through the proxy.

BUG=401244

Review URL: https://codereview.chromium.org/568893002

Cr-Commit-Position: refs/heads/master@{#296244}