Extension Support for App WebViews

It's sometimes useful to allow an extension to interact with WebView content. For example, applications running in Chrome OS Kiosk mode may want to allow users to use accessibility extensions. This doc offers a brief walkthrough for how extensions can use existing WebView and App framework support to interact with WebView content.  It describes the basic mechanisms that are available and proposes a simple architecture an app could employ to connect one or more extensions to its WebViews’ guests.