In its never-ending quest to make the Google App as useful as possible, Google has now implemented the use of Chrome Custom Tabs within the Google app so that search results links open up as a Chrome Custom Tab instead of immediately opening up in the default browser that's been set by the user, which in most cases is going to be Chrome itself. With the use of the custom tabs, links open up quicker than before which is beneficial because it saves users time, and using them doesn't have to mean that you lose out on popular Chrome features like saved passwords and auto fill.
Custom tabs includes these features too which means users have less of a need to swap over to using Chrome to view a link or open up a web page. Basically, Chrome Custom Tabs allows users to open up links and webpages from within the app that they're already using so they don't have to exit out. Google implemented this feature into Google+ earlier this year so links could be opened up right within the application, and Twitter uses the same feature for links in their Android app as well.
Although the chrome custom tabs allows users to view links without leaving the app, the option is still there to open a link within the Chrome browser if they wish to do so. Custom tabs also have options to share links, copy links, and find in page, so there isn't much that users miss by interacting with the custom tab as opposed to viewing the linked page inside of the Chrome browser instead. Users will need to have the most recent version of the Google app for Chrome Custom Tabs to be a useable feature, which should be version 5.12. To enable Chrome Custom Tabs, navigate to settings from the Google app, tap accounts and privacy, and there should be an option to toggle on or off the ability to open web pages in the app. If it's already enabled, then links will open inside of the Google app going forward, toggling this off will cause links to open in Chrome just as they did before.