iOS: On an iPhone or IPad, When you click on a link in an email or app, you're taken to Apple's Safari browser -- you have no choice in the matter. However, the latest Google+ on iOS update shows that there is a workaround app developers can use to open Chrome instead.
If you click on a link in Google+ posts in the iPhone app, they will open in Chrome if you have that installed. The Next Web explains that this is because of a URL scheme tool Apple gives to developers so they can have their apps post to other apps. The Google+ app uses the URL scheme to open up links in Chrome.
TNW also points out that Google has documentation for developers on how to use Chrome URL schemes. This means that not only are we likely to see integration of Chrome in other Google apps (such as Gmail or YouTube), non-Google apps should be able to open up Chrome as well if they wish.