When I\'m viewing the downloaded resources for a page in the Chrome web inspector, I also see the HTML/JS/CSS requested by certain extensions.
Not quite the solution I was after (I'd have preferred a global setting), but there is now a way to filter out requests from extensions, as mentioned by a commenter on the issue I originally opened.
In the network tab filter box, enter the string -scheme:chrome-extension (as shown below):

This is case-sensitive, so make sure it's lowercase. Doing this will hide all resources which were requested by extensions.