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In the event of network error, the WebSocket server response is not
validated. However, some network errors are ignored higher up in the
stack.

For example, in HttpNetworkTransaction::DoReadHeadersComplete at
http_network_transaction.cc:988, ERR_CONNECTION_CLOSED is converted to
OK.

To avoid this whole class of bugs, do not keep status code 101
(Switching Protocols) if there was a network error. It is changed to
503.

The response headers that are displayed in devtools are copied before
the change, so they will still retain the original status code.

BUG=408061
TEST=net_unittests

Review URL: https://codereview.chromium.org/889783002

Cr-Commit-Position: refs/heads/master@{#314136}