Why would I want any of this in a web browser?
I certainly understand why developers and others find it convenient and profitable to keep on adding APIs the client part of the global client-server document-publishing platform, but I am simply amazed to come across someone using security of all things as an argument or a justification for doing it.
Think about what you are saying! You have a new API you want to make available to developers, but of course it is really hard to create an API that cannot be exploited, so to keep things as secure as possible, every time the user needs to check the weather forecast, look at the new photos of the grandkids or look for a new recipe, some random web server plus zero-or-more third-party ad servers should get a crack at exploiting the new API!
Source? Seems like a long stretch, especially if you factor in non-windows desktops and all mobile devices.
If you really are concerned about security wouldn't you rather trust basically just Apple and Google or Apple, Google, and every native app developer in the app store?
Having a load of random apps installed on your smartphone is simply less secure than than just using that functionality within a sandboxed and standardised web browser that goes through endless routine security updates.
Its much easier to chose what to install from an app store. If you click on the wrong link, or if you receive malvertising, boom! vulnerabilities in this could cause your phone to be infected.