Move custom scheme registration to CefContentClient. This works around a problem on Linux where the zygote process  has no CefContentRendererClient instance at the time that CefContentClient::AddAdditionalSchemes is executed (the zygote process is later forked to create new render processes).

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk@1271 5089003a-bbd8-11dd-ad1f-f1f9622dbc98