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Tests communication between WebUI page and the native code.
When page is loaded it sends "syn" message to the native code,
when native code receives "syn" it replies back with "ack",
when page receives "ack" it replies back with "fin". Once "fin" is
received the test succeeds.

BUG=567809

Review-Url: https://codereview.chromium.org/2006273005
Cr-Commit-Position: refs/heads/master@{#396972}