Google has posted a blog post detailing the issues with plugin support in Chrome for Metro UI in Windows 8.
Google is already offering support for Metro in the dev channel builds of Chrome. The work is ongoing as currently, the Metro edition is very crude in nature.
Google said that they are only supporting two plugins in Chrome for Metro right now… Adobe Flash Player and Chrome PDF Viewer. Both of these plugins are delivered through pepper technology which works with Metro.
Other plugins like Silverlight requires an older technology NPAPI which is not being supported by Microsoft in Metro. Google spoke about these plugins:
We’ve noticed that other than Flash and PDF, usage of plug-ins has been steadily decreasing over the past few years, to the point where a relatively small percentage of our users load any of these plug-ins at all. We expect NPAPI plug-in usage to continue declining over time, especially since plug-ins can’t run on most phones and tablets. If the trends continue, we look forward to the day when NPAPI can retire peacefully to the countryside.