"View As MIME Type" in Chrome or Firefox [closed]
Shameless plug: I just published a (free) Chrome extension to do just what you ask. It's available on the Chrome web store. It works by listening to the chrome.webRequest.onHeadersReceived event and patching in a custom content-type HTTP header. If you'd like the build it yourself or see how it's implemented, the source is available on GitHub.
For Firefox, there is an add-on provides almost the function you wanted: https://addons.mozilla.org/en-US/firefox/addon/force-content-type/ . No idea if there is a Chrome extension or not.
Even if the functionality exists, I wouldn't recommend you to use it in your example: Arbitrary HTML would have access to google.com domain for cookie and script, which is really really bad in terms of security.
Ubuntu 12.04 has an extension to the System Settings called Tweak. This has a FileType Manager.