This page provides an overview of Chromium's support for out-of-process iframes (OOPIFs), which allow a child frame of a page to be rendered by a different process than its parent frame. page 
OOPIFs were motivated by security goals like the Site Isolation project, since they allow a renderer process to be dedicated to a single web site, even when cross-site iframes are present. OOPIFs are a general mechanism, though, and can be used for other features than security (e.g., the <webview> tag in Chrome Apps).
Supporting OOPIFs required a large architecture change to Chromium. At a high level, the browser process now tracks subframes directly, and core parts of the browser (e.g., painting, input events, navigation, etc) have been updated to support OOPIFs. Many other features in Chromium now combine information from frames in multiple processes when operating on a page, such as Accessibility or Find-in-Page.