Google Chrome and AVG AntiVirus have been in what appears testy communications over an AVG AntiVirus extension called "AVG Web TuneUp" which Google pointed out adds numerous Javascript APIs to Chrome with the purpose of hijacking search settings on a new tab.
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According to AVG it attended to the issue, but google has decided it won't permit AVG to auto-install the extension for new users while the Chrome security team take a closer look.
This was a significant security issue, but it now appears fixed.