Cocoa interfaces are typically created using Xcode's Interface Builder. This program produces XIB files, which are XML representations of a user interface. Despite being XML, the files are fairly opaque and should not generally be edited manually, and they often do not merge cleanly via source control. The format and content of the XIB file will be different depending on the OS version and devtools (Xcode) version that is used to edit them. If you edit files with consistent versions, the diffs remain marginally readable. Because of this, Chromium has a policy that requires all XIBs to saved with a specific developer setup.
Mac OS X: 10.9.5
Prior to Xcode 4, Interface Builder was a standalone program that could be used to edit XIBs independently of the Xcode program. In Xcode 4 however, Interface Builder is now just an IDE component, not a separate program. The chrome.xcodeproj is humongous and brings Xcode to its knees, making it unideal for editing XIB files. To make things easier, Chromium has a special Xcode project just for editing XIB files.
Any custom NSView, NSWindow, NSWindowController, or NSViewController class files should be listed in chrome_nibs.gyp. This will allow you to create new IBOutlets and new IBActions for classes, as well as allowing Xcode to maintain existing connections.
When it comes time to prepare your changelist, you should list explicitly the XIB changes you have made. This allows your reviewer to know what was done, and to, if necessary, repeat the steps if the change were to be merged or re-done. Example:
Don't add new XIB files. XIB files are difficult to maintain, difficult to review, and difficult to merge.