The idea is to enable users to check whether extensions act as they're supposed to
It seems that Google is not done tweaking the Chrome UI, the toolbar in particular. After dropping the iconic Wrench menu and the favorite/bookmark star button, it is now adding a new indicator to the OmniBox.The feature is not enabled by default and is only available in the cutting edge Chromium and Chrome Canary builds.
The new Script Bubble icon is designed to make it easy for users to see what extensions affect the page they're currently viewing.
The aim is to both improve Chrome security and stability, by giving users more control over their browser and extensions.
They can check to see which extensions are modifying the page they're visiting, making it possible to spot anything that they weren't aware of, for example one extension running on a page it's not supposed to.
Google has built several safeguards so that extensions don't do more than they claim to. But many simply ask for as many permissions as they can to make sure any change they make in the future doesn't affect them, knowing that people will install the extension anyway.
It's a nice addition for the simple fact that most people rarely read the permissions needed by extensions, apps and so on. People just click through to install the extension they want to install.
While most extensions in the Chrome Web Store should be safe, it's not always the case. And even the ones that aren't malicious may surprise you by springing up into action when you're not expecting.
The feature is experimental and may or may not make its way into Chrome. Google has always prided itself on its minimal UI and doesn't take lightly to adding more buttons or controls.
You can check out the new feature in action by running Chromium or Chrome Canary with the "--script-bubble-enabled=1" flag.