I'm having an issue where an older version of Chrome iOS 43 is being updated to do Chrome iOS 50. I think iOS is doing this automatically in the middle of the night. I hadn't touch the iPhone 6 since yesterday afternoon, but this morning I noticed Chrome iOS updated to the latest version. I need to use Chrome iOS 46 or older because Chrome iOS 47 and newer changed over to some different engine. The Data Saver feature is gone and I rely on it to keep my monthly bandwidth allotment in check. There must be a way to restrict updating apps automatically?
Turn off automatic updates: Settings app > iTunes & App Store > Toggle Updates to off.
You will have to manually choose to update apps in the future from App Store > Updates tab.
I'm not sure that Google got rid of Data Saver though. How to enable it on Chrome for iOS might be a different question you can ask.
Another couple things you might want to consider are using Safari with an ad-blocker. Adguard is the one I use and it prevents ads from loading in Safari, so browsing uses less data. And the other thing would be to get a data tracking app (I use smartapp) so you can have a better sense of how much data you have and are using.
And of course, use WiFI where available and make sure settings in other apps don't load as much content over cellular.